From the technical standpoint something that stays up potentially forever is worse than something with definite lifetime (because the former can build up over time). That supports the idea of expiration.
From usability point of view I can see potential problem. Now you can set two accounts - one as cold storage, second as active. You set your active as proxy for cold storage, you delegate all HP from cold to active and you are good to go, never to have to touch the cold storage (well, until renewal of proxy setting once a year).
Back to technical side. It would require additional index on delegation objects, so it is not only pluses in that field.
I think we'd need to see what is average effective lifetime of delegation (with and without weighting it by its power) - maybe it is not such big of a problem after all?
At this point, I just wanted to throw the idea out there and see what people think. Certainly don't want to burden anyone or suggest bogging performance.
I see your potential problem and would counter it by delegating renewal rights from cold to active. Easier said than done. I know...
And I'm not really looking at it like it's a major issue, I just know it could turn into one. There are little pockets everywhere; seemingly forgotten delegations. And even being small I view as a problem because that just adds to the total of wasted resources. Similar to shrinkage, in a sense. Especially problematic in the long run, and if price goes up.
Handing out RC, set it and forget it. Incentive to powerup; set that and forget that too. Not a fan of the free ride.
Any single one of these little 'projects' springing up asking for delegation could stop paying out whenever they feel like it. Meanwhile inactive account doesn't notice for two years, passes away, or loses keys. That little project becomes a parasite, forever. Ill-gotten gains.
Again, I don't want to convince anyone it's a problem that needs an immediate solution. Just something not worth ignoring completely. Unfortunately I don't know a damn thing about what's under the hood of the blockchain. I just drive the damn thing and want it to be running efficiently.