is it safe to just do nothing until an update with a version number exceeding 10.3.2507 comes along?
Difficult to say for sure without more info from Avast on what the fault is!
I also have a Win7 machine that doesn't seem to be experiencing symptoms except for the 21 day thing, and is in regular contact with the cloud and the definitions are current so I assume the service is OK. I'm going to leave it installed as it's my sandpit test box anyway and it
should update when >10.3.2507 arrives. Depending on how many machines you would have to visit to uninstall and reinstall 10.3.2505 might sway your approach too (ie, you might choose to fixup only mission-critical machines to ensure the service doesn't randomly stop).
I think downgrading the release number was a a poor choice for Avast since 10.3.2507 was announced (via this thread) and began deploying, however I like to think they had a very good reason to do so. But then to withdraw it and replace with a new build again without any warning on what the issue was so the unlucky ones can take appropriate action, doesn't fill me with much confidence...