Lets be real here, nobody is paying for full sub for classic anyway... yall get retail in it aswell so even if you wanted to you cant pay ONLY for classic. :P
So technically its 2 mmos in 1 subscription...
What most people completely misunderstand when saying "why play it for full price" is the fact that you arent paying full price.... if you were you would ONLY get classic.
But instead you are getting 2 different versions of World of Warcraft... so in that subscription you are getting twice what you used to get.
Literally subbing for real vanilla 15 years ago gave you less content than what you get when you sub for it later this year... its your decision not to do all that content, much like many choose between PvE or PvP and just as many do both.