Kenner made over 300 million figures during the lifetime of the run (
https://en.wikipedia.org/wiki/Kenner_Star_Wars_action_figures). Army builder collectors won't make a serious dent in that total, and focus collectors and variant collectors not at all. None of those groups accounts for a significant percentage of collectors as a whole, nor their figure collections for a significant percentage of that 300 million total. That's like blaming squirrels for the shortage of oak trees because they eat acorns. They do eat them, just a minuscule proportion of the total produced by oak trees each year.
The same might not be able to be said for MOC focus collectors because MOC's have (obviously) always been far, far rarer than loose figures because most kids opened them

. The Boba Fett tax is legendary because he is a very popular character and a relatively disproportionate number of collectors have a Fett MOC focus, and that naturally can only ever drive up prices as always happens in any supply and demand situation. I do not own a genuine Fett MOC, but I'm not (and I'm not suggesting that you are either) going to moan about it, because it's a free world and no laws are being broken and no-one's arm is being twisted into buying this stuff. It's very much a luxury purchase, and if some collectors want to focus on collecting the more popular characters, that's completely understandable and entirely predictable.
What is certainly true is that as the children of the 70's and 80's have grown older and older, more and more of them want to recapture their childhoods through the vintage Star Wars toys they owned as kids (and those toys they never had! :-D), just like their fathers did by collecting Dinky, Corgi and Hornby, etc. The rarer figures (i.e. the ones produced in the lowest numbers like the last 15 / 17 produced, or early figures subject to running changes like the vinyl Jawa and blue Snaggletooth) have always been precisely that; rarer, and consequently more expensive to buy than the average figure. That's always been true, the only thing that's changed is the number of people chasing those figures, which has massively driven up prices.
In the early 90's a loose vinyl Jawa could be bought for £50 (which was a hell of a lot more expensive than your average Star Wars figure cost back then, including Yak Face, a mint example of which might set you back £25 at most). These days of course you'll be lucky to find a vinyl Jawa for under £1,000. That's not due to focus collectors, army builders or variant collectors, but simply regular collectors who deem it to be part of the main figure run, and consequently want one. And since a significant proportion of those collectors are 40-50 something adults with some disposable income, they chase the limited supply of vinyl Jawas, driving prices ever higher and higher. And the same thing happens, albeit to a lesser extent, throughout the entire vintage Star Wars toy line. When I started collecting in the 90's you couldn't sell a Klaatu MOC for even £4, as nobody wanted them. These days the exact same MOC's go for 20 times that.
So I don't (and wouldn't) blame the very small number of army builders and focus collectors, and certainly not the even tinier number of variant collectors, for limiting figure supply, as that would not only be scapegoating, but also failing to understand the true situation, which is simply the sheer popularity of the hobby as a whole which seems to have an ever-increasing number of members chasing a finite supply of items.