In the spirit of good faith, here are my fears and weaknesses.
People mistake system-specific questions for rarely-encountered systems as either system-agnostic or dnd-5e questions about once every couple months. (Here's a question about putting armor on in combat. Here's a question about mitigating critical hits. Here's a question about a paladin using Lay on Hands on himself. You'll need enough rep to view deleted answers, though.) While that doesn't sound like a big issue, those systems might only be asked about once a month.
Consensus is currently that we're allowing experts to be experts and add system tags they think are appropriate, rather than closing the question as unclear and waiting for the original asker to edit the tags. But even experts make mistakes. Somebody who mistakes a system is likely to mistake that it deserves a system tag. Especially in the case of dnd-5e, that tag is going to take it off my radar more or less entirely.
It's not very likely that a suggested edit like that is going to make it out of queue, but 2,000-rep users have been mistaken in the past and they don't need to go through queue.
I don't want to look through every dnd-5e question on the off-chance somebody edited it to add an inappropriate tag. Not only is it time-consuming but it's so unlikely compared to the total volume of questions that I'll probably be suffering from alarm fatigue and skip right over it when I check.
(Just to pull it out of volatile comments, here's the final SEDE query from earlier discussion, which shows the last date a tag was edited onto a question.)