That way they'll make money, and they can fund the service not having to shut down, and there isn't any linkrot.
Given those options, an ad seems like a trivial annoyance to anyone who very much needs a very old link to work. Anyone who still has the ability to update their pages can always update their links.
"Here's a permanent (*) link".
[*] Definitions of permanent may vary wildly.
While I generally find the "killed by Google" thing insanely short-sighted, this borders on straight-up negligence.
I will be honest I was never in an environment that would benefit from link shortening, so I don't really know if any end users actually wanted them (my guess twitter mainly) and always viewed these hashed links with extreme suspicion.
