Wouldn't it be better if lost coins were reintroduced into the network after a fixed span of inactivity?
This is madness. The first problem is how can you identify lost coin if theres a recent event that an old address move their token recently? Doing this will make Bitcoin centralized since you will remove the freedom for people to hold in long term period because their coin might be categorized as lost if they didnt touch it for long period of time.
Exactly. I mean even today you see articles about there being like 4 million or 5 million lost bitcoin because people just assume any bitcoin that hasn't moved since like 2013 has been lost, but every couple of months I see an article about how a few hundred bitcoin of some address that's been inactive since 2010 or 2011 was just moved. Plenty of the "lost" bitcoin is not actually lost but is still owned by people.
There is no amount of time (like 109 years as the OP suggests) that can pass that would make it okay to for the protocol itself to steal people's Bitcoin.
This would make Bitcoin no longer sovereign money because in its very design it would steal your money if you held it for too long. This idea would fundamentally break Bitcoin for no good reason at all. There is no difference in Bitcoin's utility whether there are 21 million bitcoin available or 19 million or 17 million.