Apple is an existential threat to the PC

It's not removing it (for now). It's sucking it up into its own vertical integrated ecosystem. Their microled R&D will benefit no one but Apple.

Apple market share and ecosystem have been growing for 5 years now.
How is Apple rolling its own silicon and operating systems removing investment in technology? Display technologies, OLED, QD, foldable, seems to be higher than ever, as does investment in improving network technologies.

Can you point to some actual evidence of investments reducing, and where you can, can you pinpoint Apple being the cause? Because I've see no evidence at all and you're been plugging away at this narrative for five years now.

Seems like pure fantasy. Apple tends to have a highly profitable but small-market-share business. I haven't seen any industries that overlap with apple slow down.
I believe Mac shipments are down, just not as much as PCs. But there's been a lot of discounting too, of Mac laptops in the last 6 months or so.

As long as they price most of them well over $1000, they won't gain enough market share.

Same thing with iPhones, they skim the top of the market, those willing to pay more. But there are fewer such consumers.

Obviously they can be fantastically profitable with this strategy, targeting the more affluent buyers.