There are better ways. Patreon and the like are the micropayments that Scott McCloud has been talking about for decades.
Really? Personally speaking, I'd rather have some ads on the SRD than have to pay a monthly fee to access it.
And yes, it would have to be for accessing it - it's the SRD for a system that came out 13 years ago, there isn't going to be much new content for people to sponsor. I mean sure, there's "everyone can access it for free, people just voluntarily donate", but that relies on there being enough big patrons to cover it, and that's by no means a guarantee.
I should add - in this case, it isn't even about the cost, it's about being able to, in the middle of a game, without having my computer around, ask someone to look something up there and they easily can. So even if it was $0.25 a month, I would rather have ads.