Microchip seem like a likely buyer to me. They have been buying up various silicon semi-specialty companies like mad over the past 10ish years and they already have some RISC-V SOCs for sale.
If Microchip could offer a semi-custom chip design business leveraging what SiFive have done, with reasonable minimum order quantities, I think that'd be quite interesting. But I doubt Microchip will pay a billion dollars for such a purchase, they could also probably just go hire away the key engineers for a lot less.
MicroSemi (which was later bought by MicroChip) had a relationship with SiFive and you could buy a MicroSemi PolarFire FPGA dev board with a preconfigured SiFive U74 soft-core in it. DAMN if it wasn't difficult to configure.
Which is my way of saying... yeah... Microchip buying SiFive? Stranger things have happened.
If Microchip could offer a semi-custom chip design business leveraging what SiFive have done, with reasonable minimum order quantities, I think that'd be quite interesting. But I doubt Microchip will pay a billion dollars for such a purchase, they could also probably just go hire away the key engineers for a lot less.