Allowed, me thinks. Just like today you can drive antique cars on the road that don't meet today's regs and standards, there will be too much pressure.
But I doubt your premise that in only 20 years only computer-driven cars will be sold--I don't think the technology will saturate all communities, all roads, all vehicle types that quickly. Nor do I think that technology will saturate all cars by then either, my belief. Farm vehicles, semis, buses, motorcycles etc won't have handled all the practical matters of autonomous driving and they have use of the same roads. I do expect that some metro areas will be so traffic burdened in the next few decades that they'll put significant tax (toll) on analog vehicles (such as semi trucks) at certain times to try to discourage their erratic human ways.