Personally, if this delay idea was implemented I would leave the game.
Glad to hear some overall positive feedback on the game. On your last point its simply me airing an idea. Its not currently possible to implement it, if it was to be implemented it would be a current world "go-forward" basis. That means only new worlds would have the change. The function of the delay as you are calling it would be two fold.
First it would reflect real world downtime averaged out lets say over a month or a year and worked out as a percentage of total flight time. So lets say for arguments sakes that the total maintenance downtime came to 10% of flight time for planes, that would mean that you could fly your planes for 21.5hrs a day and the remainder would be maintenance time, (10% of 21.5 = 2.2, 21.5+2.2 = 23.7). Overall it would be a minor inconvenience, but would reflect real world operations.
Second it would help the game "pace". By making the game slightly more difficult in terms of max hours on an aircraft coming down it would mean people could not expand as fast, and thus keep a long running game slightly more challenging. For the majority of users having a challenge in the game makes them play more, we know from experience that if money can be made too quickly the majority of the user base would get bored.
The system, if it was ever to be introduced would be automated and the user would not be required to do any additional work.
In terms of taking planes out of service for maintenance, I'd be in favour of a hotswap powered automatic aircraft switch. So additional aircraft would be available in the fleet to cover maintenance, and as maintenance of an aircraft comes up, it is automatically swapped out, again with little input from the user. If that type system was added it may have potential as a premium feature however.
Just to conclude, its good to have some structured debate on what shape maintenance should take within AM. The majority of items above will not be implemented in the near future but ideas can be generated that will be considered at a later date.