what's the point, evolutionarily, to self destruct after reproducing?

There is no point, evolution is about successful reproduction and everything else is just random chance.

If a evolutionary tweak happens that gives your off spring better chances, but your arms fall off after sex then it'll probably perpetuate.

Essentially their entire mating cycle is what causes this. They've got a gland behind the eye that puts them into mating mode and once it starts it never turns off until they overdose on sex hormone.

Most cephalopods are voracious hunters that eat and eat to grow big and then once mating mode switches on they just focus on mating, which results in a shit ton of babies. Every step of that cycle has an extremely high mortality rate resulting in strong selection pressures for the best of every phase. When they do something, they go big.

There's a specific life history strategy called semelparity, which is what you're describing (breeding once then dying). To my understanding, this is incentivized if the chances of getting a second attempt to breed are too low, and so it becomes more evolutionarily advantageous to simply go all out on the first attempt

To prevent decrepit politicians who already had their chance from usurping the resources of the next generation and pulling up the ladder behind them?

You know... Octopus politicians

Not everything in evolution ends up having a point. So long as a problem does not impact the propagation of children it can end up moving forward to the next generation.

I would guess that if there is an Evolutionary reason, it's probably that octopi with this drive reproduced More than octopi that didn't.

Evolution doesn't care what happens to you after reproduction because you've already passed on your genes at that point

whatever works...

Are you sure it's not because of chronic exhaustion for several months in a row instead of the sex?

Octopuses didn't get post nut clarity.

Octopi is the oldest plural form of octopus, coming from the belief that Latin origins should have Latin endings. However, octopus is not a simple Latin word, but a Latinized form of the Greek word októpus. Consequently, its “correct” plural form would logically be octopodes.

Nowadays "octopodes" and octopuses" are both acceptable, the latter being more regularly used.

"Squid" on the other hand isn't Latin or Greek, of unknowns origin, probably from a sailor's variant of "squirt"; late 15c., squirten, squyrten "to spit water from the mouth" (intransitive), a word of uncertain origin, perhaps via Middle Dutch or Middle Low German, probably ultimately imitative.