So let's break this out a little...
1. Why would people get mad if WotC decided to make the SRD for their future game closed as opposed to open? The OGL, the 5th edition CC all still exist... so what exactly would the anger be about? This feels like it's edging dangerously close into the realm of entitlement IMO.
2. Less interesting is purely subjective. But yes it would by it's very nature reduce the amount of support it gets... which does not necessarily reduce the amount of money it makes. If WotC can get enough people to buy into their ecosystem of DnD Beyond, VTT, etc for a subscription... they eliminate or at least reduce their dependency on supplemental books selling a certain amount (think Xbox and gamepass here). As opposed to having to deal with unpredictable spikes and drops, predicting what the market wants or doesn't want with supplements, and also the competition in the realm of supplemental material with those same 3pp's that are supposedly making them more money... they have instead created a relatively steady source of income that they can depend on and more importantly predict... that probably exceeds what they were making off the selling of individual books with the possible exception of the core rulebooks (which are going to sell regardless). They may make more money off a single book hjere or there but overall by tying people into an ecosystem they are making more over the longterm and in a more consistent manner.
2a. Now the above just touches on the games but the fact is that it will also eliminate the possible competition that might arise from everything from knockoff D&D videogames to knockoff tv shows and movies... in other words brand control. People claim WotC controls the brand but do they when a group like Critical Role has for all intents and purposes become synonymous with Dungeons and Dragons at this point? Vox Machina on Amazon is the Dungeons and Dragons tv show in everything but name. I don't think this is something even the creators of the OGL took into consideration, the fact that it could very well lead to loss of control over the actual brand even if it wasn't out of print or being mismanaged. Short term, the effect Critical Role has on D&D is great but long term as the brand is identified more and more with their particular show (and the Critical Role brand) that's not a good thing from Hasbro's perspective... and most definitely can lead to loss of money or even brand confusion through too strong of an association if someone/something like Critical Role branches out into their own ttrpg.