This will be my fourth winter with a smart trainer and Zwift. Before that, I'd ride outdoors when the roads were dry and temperature was above 35 and it was light out. It wasn't that much fun below 40 here in Maryland, started riding less in the winter as I got older.
Now, I'll do Zwift indoors if below 40 but still ride outside above 40. To be honest, there are definitely times when it is above 40 but windy or gray and I'll just do indoor training - Zwift has made me soft!
From a training perspective, my riding indoors tends to actually be better training - I do workouts, push my power on many rides, do races, etc. Outdoors I don't have a power meter, ride longer rides for run but rarely in upper power or heart rate zones when all bundled up!
In the winter of 2018/2019, I planned my indoor training for an early flat century ride - I was in the best shape of my life! Then I continued to mix in indoor training, loosely following workouts in "The Time Crunched Cyclist" to train for the Seattle to Portland 2 day 206 mile ride in July of 2019. I did 123 miles the first day, my highest daily mileage ever and felt great on both days.
This year there is nothing to train for, but I've been doing the indoor trainer on rainy days, will use the 40 degree test over the winter again.