If the insider preview were actually a beta test I'd agree with you.
you forgot one simple thing - it's not 2005 around. and we are not talking about 'ship once, use till the end' standalone software.
it's age of online services, SaaS, constantly updated local or centralized software. and if you look at any big release of those - they often look like beta at most X.0 versions, taking days or even month to get to X.1 - X.3 version that is quite stable and feel final. just to get to (X+1).0 in half of the year.
the 'best' of it you can see at
any MMO game, either with first release or large add-ons. you never see when actually beta ends - it's never planned - you just get your invite, than almost everyone you know has one. than it's still beta, but no invite is needed. and ages later, you just notice that there is no BETA label there anymore, and they did publish something in the news about it, but you don't really care - as you would play as you did, store is same as it was before, eating your money, and most of things just works.
same stuff happens with windows. they are just different from usual OS beta versions - opening gates soo early, that it gets stable 2 month
before the release, instead of 2 month after (apple way/2nd SP rule/can't wait new android with fixed battery life/etc), and making it so easy for so many people to join the beta.
they did
more than beta, letting us vote, 'decide', and try on us different stuff and approaches, but it doesn't mean it didn't have all that beta is.
it's new age, new marketing and engineering, new way of making beta - so that they pulled out new name for it as well.