The problem with that theory is his numbers are steady. Her's are steady/going down. They're both losing undecideds to 3rd parties (or no vote at all). People aren't switching camps.
I just don't see where you got that argument from (although I can agree with your conclusion that people aren't switching camps).
If you look long term, over the last 13 months, BOTH of their numbers are steady. Clinton has been in a 46.8%+-3.7% band since Aug 19, 2015 in the RealClearPolitics average. Trump has been in a 42.0%+-3.7% band since Aug 16, 2015. Considering that a typical margin of error is about +-4%, that works well with the theory that they are steady.
If you look medium term, over the last 4 months, then Clinton has gone UP from ~44% to ~48% and Trump has gone UP from ~40% to ~44%.
If you look short term, over the last month, then Clinton has gone UP from ~46% to ~48% and Trump has gone UP from ~42% to ~44%.
These are two polarizing candidates and people made up their mind ages ago. Even if you look at the 4-way polls with Johnson and Stein added, they aren't moving much either. There are a few undecideds that have mostly split between Trump and Clinton recently. But that is about the best you can say for a movement. I see no "losing undecided to 3rd parties" as you claimed. I don't see her numbers going down as you claimed.