Is it time for Apple to “dis-arm” and create its own in-house ISA?
They don't have to. Apple has an Architectural license. They have rights to the general instruction set design and some critical associated patents, but Apple is mainly not buying almost fully flushed out IP designs.
Apple wouldn't need a 'from scratch' , in-house ISA. If they had a Arm v9 Arch License they can very likely just keep using it. and maybe drop in a few , very narrow , extremely specialized instructions at the edges if the Arm ISA and just continue on. (e.g., Apple has AMX which isn't a standard Arm standard facility. There are a few others. )
Small embedded stuff that end users' applications never see ( e.g., the SSD controller CPU cores) Apple can just flip with some internal only transitions.
Arm v10 , v11 , v12 , v13 would come and Apple could/would gradually 'fork' from them. If each of those is 2-3 years long evolutionary path, then that is 7-12 years until might have to do something.
There is a ton of stuff in the Arch Licenses that Apple already has. ( especially if picked up this initial v9 ). The future increments are likely to get smaller and narrower. Which means Arm is going to loose a decent amount of leverage. v9 has nested virtualization , a very good solid SIMD mechanism in SVE2 , and decent security updates. There is not tons missing for a smartphone or Mac there. Arm's GPU , internal chip networking , and some other uncore stuff Apple really doesn't need much of at all.
if a large enough group of Arch License holders got together and cooperated then they could just 'by-pass' Arm for post v9 collective 'updates'. ( Google, Samsung , Qualcomm , Apple , Amazon , and Mediatek would be enough. It isn't like Arm would have to loose dozens off their client list to loose control of the smartphone market. Even smaller group in the nascent server market foothold they have. )
If Son/Softbank do some stupid stuff for an extended period of time , then they'll probably be forced to give up Arm for a more sensible price ( if drive the company into large losses ). At that point Apple and some other large players could perhaps take Arm private at some sane price . Get rid of the irrational owner and Arm can go back to being a sensible Contract R&D firm that it really is at heart. ( this whole meme stock price chasing stuff is a big mismatch to what Arms core business really is. )