The raid weapons are technically no longer missable in... 10-11 months or so from now, but they would cost a resource that's usually used for something else. Treat them as permanent missable in practice.

A bit more detail on that eidolon exchange shop:
1 copy of a raid event eidolon costs 600 orbs
1 R eidolon is worth 2 orbs (you can limit break them to save space; 1* = 1 copy's worth so the rate remains the same)
1 SR eidolon is worth 30 orbs
1 event SSR eidolon is worth 150 orbs
1 gacha SSR eidolon is worth 750 orbs

If you have the space for it, you can hoard the extra SR eidolons from disaster raid farming, which is something you're expected to do anyway in between events, as well as during advent events. Granted, don't expect many advents in the near future :/