I can only directly comment on my own experiences pertaining to this issue, but I can say many of the issues already mentioned and discussed are what have driven me to not spend much on this game since the middle of 2017. Now while only wargaming knows for sure if their global sales across the board have dropped, I'll go ahead and add my two cents from a player that's not only been around a bunch, but spent a bunch as well. In no particular order:
Matchmaker has screwed over the main premium tiers, i.e. V, VI, and VIII.
Being bottom tier can be a fun and challenging experience, but playing a Texas or an Othoknik in a tier VII match is asking to be punished. While some ships can hold their own while being bottom-tier (looking at you, OP Guilio Caesare), more often than not it's an exercise in futility. Grinding out tech tree ships is a necessity for a lot of players. Why choose to play at those tiers when I can actively avoid it? As such, it's skewed my premium ship purchases away from anything in those tiers. The first thing I look at when a premium ship is leaked is 'what tier is it?' Unless it offers something unique or different, I'm not touching it.
The premium ships released offer nothing interesting or are broken in some way, shape, or form.
Either these ships are underpowered or are so niche as to be generally unplayable in most circumstances. Take the tier VII Duca deli Abruzzi. Weak guns, weak armor, and nothing else of interest. Even with tier VII MM, why would I buy this? Take Asashio. It does one thing exceedingly well (nuke battleships). It offers nothing else of interest and cannot carry a game with its gun and torpedo armament. It can out-spot destroyers, but otherwise will have a hard time winning a gun fight with one. And the thing can realistically be killed by a tier III cruiser (pick one. Maybe not Varyag though). So why would I buy this? Take the most recent Z-39. Four slow-firing guns doesn't give me confidence. I'd rather play the Blyskawica with its seven slow-firing guns.
Captain experience has made using premium ships to retrain redundant.
Back in the day, captain experience wasn't a thing, so using a low-tier ship to train up the lower skill-point captains was almost a must to prepare them for high-tier gameplay. This is the reason my Texas has so many games in it, because all of my cruiser and battleships captains came up in her and were retrained in her as they moved to other ships. Now? A new captain gets slapped with a boatload of elite XP to get trained up, or if I need to retrain him for another ship, drop the 200k silver to cut the cost in half, finish retraining with the discounted elite xp, then throw the boatload of xp onto to him to get him where he needs to be. It's cheaper and quicker than playing a boatload of games in (pick a ship).
I've personally already spent more on this game than I care to admit.
Doubloons, flags, camos, ships, premium time, you name it, I've probably bought it (not the Krasny though). The reason I bought everything was to expedite the grind (as intended by wargaming, no doubt). Part of the grind was for silver to afford the ships, another was the grind for captain xp, and lastly the grind for the ships themselves. Silver isn't a problem since I have the free experience ship Missouri, plus other credit stalwarts such as Atago and Tirpitz. Buying additional premium ships to generate money isn't a requirement anymore. Captain experience I've already hit on, thus negating the need for nation specific and ship class-specific captain trainers, leaving only the regular ship grind. Premium ships do not directly help with this grind any (save for generating a paltry amount of free experience per match before flag and camo modifiers (yes, I can generate a metric ton of free xp with the proper flags and camos, but I can do this with any ship, tech tree ship included)). Therefore, any money I spend in the future isn't going to speed my progress up any quicker than it already is, nor will it make the game any easier (pay to win arguments need not apply here - I'm not going there and this isn't the thread for it). Since throwing my wallet at the screen isn't going to speed things up, the only thing I'm going to throw my wallet at the screen for are ships with historical significance, something unique that isn't a "garbage-bote," or some combination of the above that's fun to play. Current and recently introduced ships have not checked those boxes for me.
Give me a deal, Wargaming.
Your bundles are overpriced and full of stuff I don't want/need. The ship plus port slot by itself is nice, but the cost is too dang high for what the ship is in my book (see above). Since I started playing back in August of 2015, my life has changed a bunch. I'm back in school. My disposable income as dropped. As a result, my time to play the game has dropped too. I can't justify a fifty-plus dollar purchase of a ship I'm hardly going to play, especially if it does nothing for me. The steam deals were great and I scooped them up despite not needing the ships nor extra doubloons. Those have been my only purchases in 2018 thus far, and I don't see my wallet opening up anytime soon for anything on the horizon. Charging $14.53 for a ten-point commander with some small bonuses is steep, plus I already have their "brothers." The new ships on the horizon seem lackluster (no interest in an Argentinian cruiser or another USN DD at tier VI. Maybe Cossack if it gets out of development purgatory. Maybe). You know what is a deal? Those Santa crates. Yea, it's an RNG fest, but if I throw 100 bucks at the screen there's a really good chance I get the premiums I would like to have and at a huge discount. And they come pre-loaded with a 10 point commander, whereas the basic bundles in the store now do not. That's a deal, and I'll gladly shell out three-figures for a chance at multiple ships, the bonus camos, the captains, and any other goodies I get along the way. Instead of buying things now, I'll wait for a chance to get them at pennies on the dollar. That's a deal worth waiting for.
So in short, buying new ships does nothing for me. I'm also at the point where I'm not even excited for their release anymore. I remember being stoked for when the lackluster Hood came out. As a non-CV main, I was thrilled to see Enterprise hit the shelves. Prinz Eugen pre-buff was cool too for historical reasons. I was excited for Roma, tier VIII plus wonky guns plus huge price tag kept her away from me. As it is, I have 157 ships in port and counting, and adding another premium I likely won't play much isn't a wise financial choice. As others have pointed out, playing fifty games in one ship to get my 'money's' worth seems excessive. Unless something changes, I doubt my spending habits will change any.