the reason we lose flu immunity is because the flu virus mutates quite quickly. If it stayed unmutated our immunity would last much longer.
Not necessarily. Not by a long shot. That exposure/infection = long immunity is a
myth.
Infectious diseases interact with our immune systems in various ways. Not all of them, by a long shot, do so in a way that has use keeping up levels of antibodies that produce immunity for months or years.
Some researchers believe...
Belief is not a measurement. Hope is not a strategy.
First, we have to note that "strain" does not equate to "different immune response". You can have two strains that have the same protein markers for our antibodies to lock onto. So, "50 strains" does not equate to "need 50 vaccines".
However, for sake of argument, let us assume that there are 50 strains we'd have to tackle separately, and they are all out there. If they create lasing immunity, you should be becoming immune to one strain each time you have a cold! In the US, in K-12 schools, on average that's probably a couple strains a year - kids are always getting the sniffles. By the time you are 50, it should be darned near impossible for you to get a cold.
But it isn't. 50-year olds get colds all the time! That everyone can always seem to catch colds suggests rather strongly that there are significant limits to the immunity coronavirus infection grants.
Edit to add: Some folks will note that cornoaviruses aren't even the most common cause of colds. That is correct - rhinoviruses are the more common culprit. However, even with 90+ serotypes of rhinovirus out there, the argument still holds - if we generally generated lasting immunity to a virus, getting a cold should be difficult by they time you are an adult, because we are
constantly exposed to these viruses.
Ergo, hoping or planning for herd immunity not generated by a vaccine is not wise at this time.