I think it’s more accurate to say that a disproportionate amount of high intensity “weakens” your base rather than “shortens” it.
As far as training modes go, I suspect that a lot of high-intensity cycling will weaken your aerobic capacity if it isn’t supported by enough zone 1/2 work. How much is hard to say due to the non-weight-bearing nature of cycling and individual variations in adaptation. Cycling has less benefit for weight bearing sports, so perhaps it also does less damage at high intensity.
It’s hard to say. You’d probably have to test it somehow to find out.
In general, what’s your distribution of training minutes between high and low intensity? What’s the ratio between zone 1 and 2 work and minutes training above zone 2 (across all training modes)?