I leave the tickets as they are. I think, it won’t be a performance issue at any time as normaly a ticket also has an agent assigned, which is in fact nothing else than a subscriber.
And since closed tickets don’t get much updates anymore, it shouldn’t affect the search index, notifications etc.
The only thing I changed from the default setting, is the overview. I added the status to exclude the subscrided AND closed/merged tickets.
For me I’m trying to cycle through closed subscribed tickets regulary so that the number of tickets isn’t getting too much. Unsubscribing manually right now is a slight pain through.