Your points about this are all completely fair. It's up to you to integrate or reject the changes a PR does.
Currently the latter however does not happen in a manner visible to the creator of the PR or anyone else looking at the PR. So they just stack up looking like they were never looked at. If you consider something not possible to be added (or even not possible in the near future) everyone is better off if the PR would be closed with a short note of the reasoning. PRs do not age well and it's better to close them than letting them sit until no longer mergeable. The closed PRs (and issues) are still searchable, so if people to their due diligence they'll see that things were rejected before, find related discussion if attached, …. If you're not happy with the code for a given change you can also help people come to a version you're happy with maintaining without needing to go code it yourself. This does even work for issues. One of the most encouraging responses to issues I know from other OSS projects is "Sounds good. PR welcome.". I rarely see that as a response to bigger feature requests, but on smaller incremental things making parts of the project better it's a good way to move the burden of actually fixing something off of the maintainer themself.
If a PR is not of that kind it's totally fair to close it with something like "This is to big a change and needs upfront discussion before consideration.". You don't need to take the time of reviewing PRs in depth, when you already know you'll not merge from just the title/description/amount of changes/….
So in conclusion: Without a response from your side a open PR is dead weight in the context of the community helping you improve processwire. Someone did some work and now it's stuck. Even if the response is negative it'll give anyone involved context either to adjust and be fine with things not being added or next steps to get things integrated either by starting a needed discussion, changing the code or implementation or whatever else needs to be done before it could be merged. Given the fact PRs don't age well I also feel response should be reasonably timely – even if the merge actually happens at a later time. Take a year and the PR creator might have moved on or worked around it, the underlying code changed in the meantime so the PR is no longer easily mergeable, but the issue fixed by the PR might still be present. I know this well given I have two open PRs for processwire from 2017 – one small fix, one not as small feature. I hardly use processwire anymore by now.