could not be solr able to close oldest warming searcher and replace it by
new one?
That approach can easily lead to starvation (i.e. you never get a new
searcher usable for queries).
It will not. If there is more then 1 warming searcher. Look at this schema:

1. current in use searcher
2. 1st warming searcher
3. 2nd warming searcher

if new warming searcher is needed, close (3) and create a new one (3).
(2) will finish work uninterrupted and it will replace (1)

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