They bought Alpha to strip it for parts (Sandy bridge looks an awful lot like ev8) and because nobody else could make alpha chips which killed a competitor to x86.
The game has changed.
If they buy SiFive, that encourages even more investment into the ISA. They can’t kill it by doing that and even if that were the real goal, M1 competition would just convince everyone they were lying and working in secret.
What Intel brings to the table is talent and money. They have a ton of amazing developers worth decades of experience. They would think nothing of throwing two or three teams at the problem even if most of that money would be wasted.
Then there’s software. Intel realizes more than anyone just how important good software is to an ecosystem. A couple billion dollars for a thousand man years of developer time is also not much to them in the grand scheme of things. They’ve wasted way more money chasing actually bad ideas (see all their GPU attempts through the years).
ARM is wary of RISC-V, but they’d be actually scared if they knew Intel was about to throw massive amounts money into R&D.