The year is 2140. No one is using the first layer for transactions anymore.
This means Bitcoin is already dead. No point in discussing about coin recycling.
Wouldn't it be better to take unused coins and use them to incentivize miners to continue their business?
Would it be better to take someone else coins to incentivize the miners? Obviously not. Would it be better to take lost coins? Maybe. Is there any way to determine if a coin is still owned by someone? No. So you can't take unused coins without confiscating some coins in the process.
Besides that, if we have to rely on "unused" coins to incentivize the mining process, then the Bitcoin network would be dead sooner or later.
In the year 2140 we start stealing the coins which have been not moved since January 2009.
In the year 2144 we start stealing the coins which have been not moved since February 2009.
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In the year 8428 we start stealing the coins which have been not moved since January 2140.
...
Let us reach year 2140 without sustainability issues, and our great-grandchildren can continue this discussion, because I'm afraid there are a lot of obstacles until that year.