In matches that matter, teams will do whatever is possible to win games, which for teams defending last means doing whatever is possible to not lose the game. A class switch delay will not change this dynamic, players on last will still switch to the best classes, but it will greatly affect the counter-push if they are successful in their defense. The switch delay would only serve to stalemate the game even worse, as teams on last definitely won't be able to push out of last.
What you should really be trying to brainstorm ideas for is how to make players run offclasses in situations that aren't last stalemates. Every class in the game is viable for specific situations, the problem is that game situations change very rapidly, so one class being viable for one situation becomes terrible in the future. An idea like a 3s class switch anywhere on the map works much better at solving this problem.
And no, class limits and weapon bans are not put in place because of the way current last holds work, they are there because in situations where a particular massing of classes or a particular use of weapons completely breaks the flow of the game. The situations might be rare or they might be all the time, but either way, allowing a trump card to be played in the form of a banned weapon / class would be a huge mistake.
I don't have any plans to seriously implement this into the game.
Then why are you even suggesting this as a solution to a problem then. At least if you are going to theory craft do it within the confines of the game as it actually is or could possibly be.