It scaled pretty well compared to everything else. I was there, they were running tens of thousands of engineers and hundreds of p4 commands per second off a 16 core box with a few replicas before starting the Piper migration. Commit rates were comparable to what Anthropic report. And although you say it barely improved, they were doing that sort of scale on 15 year old hardware and the p4 admins wrote a paper where they said Perforce had improved scalability a lot over time, so it's not like the software was static. Architecturally Scion sounds rather similar.

It's always a question of opportunity cost. The culture that led to Piper also led to them creating their own IDEs and everything else, often with questionable outcomes. Google had the profitability to get away with that kind of profligacy but do the AI labs really want to go the same route?

I feel like "didn't scale well" is unfair to Perforce. It scaled to accommodate an absurd monorepo that might have been, at the moment they switched to Piper, the largest single software project in history. It scaled to handle the needs of every organization in the industry except Google and Microsoft.