Haven't there been periods where the difficulty went down?
Yup, I just mentally corrected TierNolan's proposal to match ones made by many people before have a minimum total chain work checkpoint (e.g. any valid chain will have at least 2^70 work by height XXX). That allows the same protection from spamming as the current stuff.
You are free to use and distribute a fork of the client without any checkpoints. Checkpoints aren't required. Assumming an attacker isn't attempting to DDOS your node by filling it with years worth of bogus blocks or try to perform a 51% attack months deep in the chain the lack of checkpoint will have absolutely no effect on your node.
Uhhhhhh. Or just run with the checkpoints=0 command-line/config option.