My opinions on it are very mixed. When it was kept on the 'down low' I scoffed, laughed it off and moved on, however after a a brief exodus from RPGs a while back (due to lack of time) I returned only for it to appear the landscape has changed and they've become quite prevalent. My local gaming Facebook groups have a few very loud, very obnoxious 'professional DMs' shilling as desperately as they can to weasel in every avenue to advertise, particularly as Corona shut everywhere down. Meanwhile Roll20 - already dubious in it's quality - went from quietly shunting them in a corner to being about 50/50 Paid vs Free games, complete with a whopping great red (flag?) label.
After having read a few discussions about this elsewhere, I can sort of see the logic behind the argument that players who pay are far more likely to be committed and show up: if they're serious enough about it to pay, they're probably serious enough about it to be reliable, pay attention, turn up on time, actually want to get involved and be on the ball with what's being played. Now for DMs who don't have a tight circle of friends and rely on 'da community' at large? I can see the appeal in that, especially online, where the average Roll20 player seems to struggle writing out their own name, let alone colour within the lines - I'm barely exaggerating, the amount of people who can't follow basic instructions on signing up to games as requested is beyond ludicrous. However being the cynic I am, I feel like for the vast majority that's just an excuse to justify it and most either want to make a quick buck off their DMing, or are naive enough to think they could actually have a living out of it.
What I've noticed, at least online, is that the vast, vast majority of 'Paid DMs' are literally just purchasing modules like Return to Avernus, Storm King's Thunder etc and running them straight out of the box, particularly on platforms like R20 where pretty much everything from tokens to NPC text is done, you just need to move the counters and documents around on screen. Naturally they toss things like "I invest the money back into R20 to improve the platform!" but when they're asking an average of $10-$20 a head (WEEKLY) with five (if not more) players? I find that doubtful. Most I see also demand 'back payments' even if you miss a session, which is all kinds of hilarious.
Ultimately it doesn't effect me I suppose, being a Permanent GM, but if I was a player I would be conscious in the future of it potentially being the norm. As Steven Mitchell says, for me it would also take it beyond a hobby and into something with much more, well, professional expectations. I also know from experience that a lot of players, particularly those coming off a unhealthy diet of Critical Roll, have expectations of just about everything and anything going the way they want - rerolls, enemy motivations, NPC reactions - and will have a tantrum of, "I'm paying for this, it should be like THIS!", something which I've encountered in free games even.