The problem isn't related to the country one is racing. The problem is racing and mixing with people outside of the Tour bubble. Racing the Dauphine is controllable and good. Racing Strade Bianche, MSR and the Nationals is not; there will be too many non-Tour related people involved. If more races are needed, then add some warm-up races that involve only the World and same Pro Teams as the Tour.You need racing KMs in your legs before the Tour and it's 100% impossible for riders to get that without leaving the country they live in.
No need to scrap the Tour, everyone just needs to be more creative in the lead-up. They (UCI, ASO and the teams) have more than 3 months to prepare, so it shouldn't be too difficult to get it right. But if they mess up the Tour, THEN the whole season will be lost.