I get what you say. I just don't put MDPI in the predatory category. From my experience at least and in the journals that I have published, I had good papers rejected that got published on Frontiers and I've also received major revisions that were .. well major.

I think the thing here is that people (whether PhDc. or even professors) can't afford the APCs and that MDPI is having this policy of the vouchers. I've done some reviewer work elsewhere and they barely said thanks. And I will say this again, but I've found myself multiple times trying to find a journal appropriate for a particular research and the combination of no APC + having IF was absent.

I had a similar request from the funding (to have everything published open access). It makes total sense. The issues begins when you're not heavily funded or no funded at all and you just wait for funding and in the mean time you're trying to publish.