Why bother if it's just going to be another 2-3 years for the next tidbit? All buying is going to do is give you a bit more to use up well before another book shows up.
It won't be though. There will be things like SCAG, etc. in the mean time. Also there will be tons of 3PP books. Just because you don't use 3PP books/homebrew, doesn't mean that most tables are like you.
By the time you get that many people clamoring for it, you've lost a ton of people to other games.
Says you. Their market research says differently, otherwise they wouldn't be pursuing that strategy (well unless you think that they are incompetent, but that is a whole other argument). I will further respond to this part down below.
You are (and to be fair most people do).
WoTC is playing the long game. Churning out splat will make some people happy (and probably increase short term revenue) but will also annoy others, but beyond that it will accelerate the demise of the 5E, which will cost the brand long term. Remember that during the playtest time between 4E and 5E there wasn't a whole lot of products to sell, so revenue and profit dried up, and there is no guarantee that 6E will be as good as 5E at keeping the returning players, new players, etc around. WoTC wants an evergreen edition because brand stability is very important, and that combined with everything means that of release is unlikely to change.
Managing a brand is a long-term game, and WoTC is doing it by the textbook (literally). Now that doesn't guarantee that it will work, and it definitely won't make all of its fans happy, but that also doesn't mean that they are doing it wrong, just because they are doing a different release schedule from your preferred version.
*Also I sometimes feel the way that you do, so I get it. I really want a setting book for Dragonlance, it isn't going to happen because it doesn't make business sense for them. I won't get everything that I want, but from a business standpoint it is probably a smart decision.