Not in their world view. In my context this is why i can say "the sub value is reduced"...note i did say "understandable", not "good choice". Sadly it shows where they are headed (since Vanaheim at least). Be prepared to buy for any scrap development in the future, since they obviously do not see development of content as part of the subscription value. (Actually while they developed dragonspine i was subbed and even as f2p later i was paying for other stuff, so in theory i should have had access to it as f2p, which i had not. Would it have been nice to have it? yes. DId i have a right on it? Not with the terms and conditions i signed. Did it put me off from buying content or sub that it was premium only? yes, sure it did). I am still curious if truly all "unchained" content will be available for f2p or if they do not end up in an "unchained lottery content extra hype mega pack box" ...
In theory and legally they could do similar to what you say...announce a big revamp and offer each instance (from tortage onwards) as module you have to pay for and make the "sub" only a collection of fluffy speedup and bonus stuff...charging everyone not actively subbed at time of change extra. Would that be a good choice? I think you know the answer...which frightens me now that i posted it, because they are maybe heading this way