True but isn't it better to go out on a high? If you wait until the decline then you have a marred franchise and anything afterwards could look like a desperate resurrection.
The reality is that the cast is getting older with each passing year. They're supposed to be young. Tobey Maguire is older than me for pete's sake (pun intended). Sean Connery got slated for doing the later Bond films, Roger Moore also had the same critics on him and until Daniel Craig was picked that's what a lot of people were starting to say about Brosnan. It's shit but that's what happens. Good actors shouldn't lament for they can always move on to pastures greener.
Your point isn't helped out by the fact Roger Moore sucked and almost single handedly ruined Bond for a lot of people. Plus in the comics Peter Parker grew up. (Although they have been pretty much screwing with all is character development the last few years to make him go back to the way he was for some stupid reason.) None of that even takes into account you can have different actors playing the same role without having to redo everything. It's not like all the previous Bond movies didn't happen anymore when the next one came out or even when another actor came on board. They were all a part of Bond's canon. (With one exception that I know of in past Bonds and the whole new Daniel Craig version excluded. Even then in a lot of ways the Craig verson could just be seen as a sort of prequel to the others so it might fit also.)
There is another issue that needs to be kept in mind. Rebooting something like Batman after about 25 years since the last incarnations of the his movies started is one thing. Rebooting Spiderman after a short a time is another. Lets just say hypothetically the new Spiderman movies drastically underperform expectations. What are they going to do then? They can't reboot because they have just done that, and they will have left nowhere for themselves to go. People will start writing off the movies if they have three or more reboots in that short a period of time. I know I would. By using what you have when it's still doing good and is still working, when things finally do head south for some reason you leave a way to fix it.