I am older and seen a lot of tech come and go. RISC-V feels to me like something that will be huge. There is so much momentum already. Not many things catch on that quick. But when they do they end up huge.
The size of RISC-V all depends on the decisions by the salespeople over at ARM. If they see the threat and dramatically reduce prices, ARM will stay dominant while RISCV remains a 'maybe one day' thing. If they miss the threat, the industry will switch over to RISCV, and ARM will go bankrupt. The whole RISCV project exists as a bargaining chip against ARM.
If you need a simple chip, and that's large parts of ARM sales volume there are free chips that you can fully control and its hard for ARM compete.
That said, the idea that ARM will go bankrupt is insane. Of course they will not, the market is not one big market but lots of small once and ARM has a massive head start in all of them.
I see no issue with both ISA existing for quite a long time. ISAs are like programing languages, after a certain size, they can not die.