2015 is when it started to gain steam as a community run project.

But version 1.0 of the spec [1] was released all the way back in May 2011, and the first RISC-V chip was taped out at the same time. This is 5 months before ARMv8 was even announced, and we didn't start seeing actual aarch64 chips until late 2013.

And TBH, I'm not sure anyone realised just how good of an ISA aarch64 is until quite a bit later.

RISC-V 1.0 isn't binary compatible with modern RISC-V, they hadn't frozen the encoding, but rough design is all there.

[1] https://www2.eecs.berkeley.edu/Pubs/TechRpts/2011/Archive/EE...