I think you’re looking at this through rose-tinted glasses.
In my opinion mounts, pets, toys or whatever don’t count as content, so in that case, you still get everything with the expansion and a sub.
But even if you think it’s content, back in the day you also already had Collectors edition mounts and pets, the stuff from Blizzcon (before it was digital), TCG codes, etc. Not much different than today I think.
How so? I actually think in recent years, they’ve gone away from making you log in all the time, with less daily quest and much better catch up mechanics, so it doesn’t matter too much if you don’t play for weeks or even months.