Surely aids falls into this category

let's say you had a virus that was like a really low grade cold for two months, and then spiked up really suddenly and killed the host.

You're right, after you kill that host you won't go anywhere but you will go lots of places that two months prior.

The GPs point was how long the host is a carrier before symptoms arise, let alone severe enough to incapacitate them.

What makes COVID-19 so dangerous is the incubation period + infectiousness, not the mortality rate.

We have much more mobility and contacts with other people than in past, therefore it may be more efficient for viruses to evolve into having longer incubation period than becoming less lethal.

If you limit your hosts to those having sufficiently grown up kids, or wait killing and invalidating them until they have, that can work fine.

What if you have a virus which took 80 years to kill its host and was super effective at spreading?