I think classic and retail are fundamentally different games with fundamentally different audiences. I only play classic and haven’t played retail in years. I used to come back every other expansion for a while, but I haven’t done that in some time now, and have no plans on doing it ever again. It’s just not a game for me anymore. The kinds of changes Blizzard would have to make to entice me to come back would alienate thousands more and would not be worth it for anyone, so I’ve made my peace with it.
Out of my classic WoW guilds with 60+ unique active members, I can only think of one person that actively plays both classic and retail at the same time, and maybe a couple more that would check out the game and goof around for a month or so every now and then. Vast majority of the people from my classic guild that I talked to about retail have quit many expansions ago and have no plans on ever coming back. There was even one guy that only ever played retail vanilla WoW and classic vanilla WoW, and quit before classic TBC came out, so props on him for sticking to his ideals, I guess. As for retail players, I have some friends going back to high school days who used to play WoW with me, and they still play retail WoW to this day, and out of them, only one has stuck around with classic, while rest of them gave up before ever even reaching level 30 and went back to retail. I’m sure there’s some overlap between the two audiences, but overall, it feels like retail and classic WoW audiences are two pretty distinct entities. Retail players are probably more likely to switch to FFXIV than they are to classic, in my experience.
I think Blizzard takes note of these things internally and that their findings back this up, which is why there have been cases of them releasing content for both classic and retail at the same time, while one would assume they would take turns if they believed the two games had a shared audience. If WotLK turns out to be a massive success and Dragonflight turns out to be a massive failure (or the other way around), I don’t think that would make Blizzard make changes to the game based on this kind of feedback.