That's the question. Is the higher speed designed for a possible increase of line speeds? Or just to enable faster running on the rare occasions they venture on to the fast lines (possibly when ECS)?
It would make sense to build in some future proofing for trains around for another 30 or 40 years.
I would say it's future proofing, and partly simply that the performance of the train is such that nowadays there's less of a trade-off between performance and top speed - it's quite possible to have cake and eat it too and have both.
Even if the slow lines were rated for more than 75 mph, the closeness of stops means it would likely rarely be achieved. Would of course be useful on the rare occasions they will stray onto the fast lines, however this is even less likely than today with the new timetable.
What I would like to see is the slow lines uprated to at least 90 mph. Outwards of Welwyn Garden City this really would make a difference. 365s, 387s and 700s are quite capable of pushing towards 100 mph on sections like Knebworth-Stevenage and Stevenage-Hitchin, as well as the longer sections north of Hitchin. Hopefully one day this may happen, although I suspect there would be some alignment works required in certain locations, around platforms and bridges, as well as of course resignalling.