It's not just the Welsh pace. The Byzantine pace was also about 2.5 feet. Their double pace was roughly a Roman pace. So we have multiple old paces (at very least Byzantine and Welsh) and new paces (US) around 2.25 or 2.5 feet. We also have multiple old paces closer to 5 feet, such as the Roman one just shy of 5 feet. So rather than invariably being around 5 feet, they've actually varied between roughly 2.25 feet and 5 feet. That's a variation by a factor greater than 2, which is a wildly large variation compared to things like historical feet. So anyone who's claiming 3 feet is way off from a historical pace is just a little confused about historical paces.
Meanwhile, ArM5 has defined what "pace" means in its context as 3 feet. So there should be no confusion there, except that it's a hard-to-find rule.
It could be cool to use Roman units. For those of us quite comfortable with units, used to them changing (e.g. all those redefinitions in 2019), and used to inventing our own (e.g. using theoretical units) adopting Roman units wouldn't be an issue. But that might tend to confuse a lot of modern players, which would be a problem. This is why I really like "pace." It gives this historical feel while being very similar to both the yard and the meter, the two of which cover units most of the world is comfortable with.