What are the best explanations out there for why mining rewards shrinking substantially on BTC won't create a BTC security threat in the next 10-15 years?

I believe most have acknowledged they will with no intervening action Governments stepping in for security feels more likely now than ever as a backstop

Government stepping in does for the save does feel very antithetical to the whole deal. And you'd still need on-chain consensus.

they take over mining and do it at a loss? isn't that the concern? I'll admit I'm not an expert here

tax-payers subsidizing the crypto maxis feels like a difficult long-term proposition.

Those who have heavy positions in the asset will subsidise the mining operations to secure the network, until something new/superior comes along I’m not technical but I can’t see them upgrading the network - was always confused how this would work without a hard fork