If there are as many buses as I just posted quoting from Wikipedia with an imminent growth by ten, [and that is quite likely the case] then there are plenty of buses involved compared to last year and Express makes no dent on the rest of the bus service.
If your supposition that bus drivers are pulled from the regular routes to staff the new Express routes, then that could possibly affect the regular routes. But what if the full time bus drivers are putting in 35 hours and can now do 40 hours? And what if there are a lot of part time bus drivers looking for more hours? In that case, the new Express routes will have zero impact on the regular routes.
What if this "test" this CM mentioned is some other scheduling test that has nothing to do with Express?
There's a lot we don't know for sure. But there's a lot that we can make fairly certain suppositions. And it's fairly certain that a shift of 3% of the buses during non-peak hours isn't going to throw a noticeable monkey wrench in the regular bus routes (regardless of whether one thinks they're doing well or not... they're not being affected in a noticeable way).
Doesn't matter how many buses Disney owns or how many hours the bus drivers are willing to work. There will only be as many routes being driven as Disney is willing to budget for. And none of us knows the answer to that.