https://thegraph.com/ seems to rely on the assumption that events will last forever.
But this post: https://www.reddit.com/r/ethereum/comments/a2ocy6/full_notes_from_fridays_eth_1x_sync_call/ seems to imply that logs dont last forever due to pruning.
Is the Graph doomed to fail?
Currently it's a de-facto invariant that logs last forever. If the protocol is changed to get them pruned from the network this invariant will be broken and implementations relying on it too.
Disclaimer: the rest of the answer is speculation.
We should not underestimate the rigidity of a system with load on it. So I do not think that core devs are simply going to disable this over night without an alternative for DAPPs relying on it.
I would add a word of caution to what @ivicaa said because there is a subtlety to be aware of when coding a software client (could be a server) that listens to logs.
Software clients need to be aware of transaction confirmations and finality rules because log emissions are part of confirmed transactions. The same uncertainty applies. Log entries may be reordered or dropped for the same reasons that can cause their corresponding transactions to be reordered or dropped. One should be careful to avoid treating a log event as a certain thing after a single confirmation.
Hope it helps.
