I think if I rode around in strictly L1 and L2 for 2 months and got up to 120 CTL in the process...
1) burn out
2) become very skinny (ride more, eat less)
3) be really good at riding around in L2 and not much else (use it or lose it)
4) take more than "a few weeks" of training to be able to do anything useful in L3 through L7 again (use it or lose it, redux)