Wouldn't it be better if lost coins were reintroduced into the network after a fixed span of inactivity?

I suppose the first thing one would say no for, would be by virtue of bitcoin's well-known deflationary property. This said, I believe it is much more important, especially in the longest term, that bitcoin keeps its property of conservation of energy, that is, having a fixed quantity of money ever available. By losing coins, instead, and by having them unrecoverable, we have not a fixed quantity of money available, rather a decreasing one, which is why we call bitcoin deflationary.

What would happen if we would make so that coins with 131 years of inactivity would be reintroduced into the network?

I see this having some positive effects:
- decreased impact of hereditary monopoly
- incentive to let the currency flow
- canceled the future necessity to increase the number of decimals for 1 BTC (very-long term perspective: too many coins have been lost, thus the necessity to further subdivide a coin)