One issue with that is that electricians can sometimes be brutal on Cat5 used as a phone line. I’ve had drops that were basically unusable as ethernet because some of the pairs were damaged, even though the phone pair in use was fine.

Even if not... I've managed to link at Gigabit over category 3 cable of a short distance.

Even MUCH older phone lines can push gigabit with G.fast, I have gigabit fiber internet in early 1980s construction that way.