[kakarot]

> if we're forced to discard speculative execution as a concept

Do you have any reason to believe this will be the case?

> Further back than the Pentium 4

This is a bad example.

The Pentium 4's pitfall was a high clock speed with little concurrency, and thusly a big heat problem that limited peak computation power.

Yes, we lose some concurrency without branch prediction, but post-P4 multi-core processors would have to take some very major hits to be comparable to the P4 in a modern multi-threaded computing environment.

The P4 is just not a good reference point for old processors because of its architectural differences.