Yeah this is a thought actually. I have an old 9-speed ultegra casette on my trainer bike and even that can be a pain in the ass when trying to hit target wattages. (Though I imagine that’s less of a problem if you’re using erg for actual workouts and only shifting in Zwift.
Most people can’t hold the same position outdoors as they do indoors, and vice versa. (For example, a lot of people can get super aero indoors but it might not be practical for bike handling or seeing what’s going on around you. Likewise the lack of movement on a trainer can cause pressure points you don’t notice outside.) For that reason I don’t think it’s worth chasing perfection as long as your indoor position is relatively similar- all your really race-specific workouts should probably be done on your race bike anyway.
Provided you don’t have another bike for commuting/rainy days/etc, I also think it’s convenient to be able to ride your trainer bike outside in a pinch. I’ve used mine for commuting when my car broke down the other week, rides when i know I’ll have to leave my bike by a beach or pool/in a car etc (I live in a dodgy area where nice bikes go missing all the time), and lending to friends who want to try cycling. Probably depends on your circumstances, but sometimes there’s times when you’re like “man I wish I had a shitty bike for this”