...I am unsure what you mean by 4E being so difficult to modify. Personally I find it the easiest of all the D&D systems to change and alter. I am still a little undecided with paragon paths. As a concept, I have nothing against paragon paths, as they are small and easy to create, allowing for variety and themes in the campaign world, but in the end they are largely only made up by three simple class features and three powers. Epic destinies have the same problem, except an epic destiny is kind of like a plot device in a bottle...
...Traditionally, in D&D yes, some classes, particularly bards and rogues have been the jacks of all trades; the ones with the most skill points and thus been able to perform more problem solving challenges but I don't really agree that is how it should be. I do not understand why class a should have more skill points than class b. Any character could have learned to pick locks or disguise themselves. It is should be more a question of background; what did the character spend his youth doing?
For 4e what I mean is that the system itself is codified and rigid. Look at sorcerers and wizards from 3e pretty much the same class but achieved different ways. With 4e AEDU power schedule you couldn't do that.
Making a whole new class in 3e was pretty easy. Think of a schtick, pick an appropriate hit die, advancement tables for attack bonus and saves, amount of skills and a list. Then using the schtick design a few interesting things to fill up levels to 20.
I've made plenty of classes for 3e this way and its fun and relatively quick. In 4e I've made 2 classes and only for first level. Seriously designing what amounts to a whole spell list for 30 levels worth of stuff and making sure it works with other classes without stepping on toes is a nightmare.
4e is ok for house ruling if you want to add a power here or there or make your own monsters, which I found both fun and easy. But If I wanted to make wholesale changes to rules I found it everything was just too interwoven and didn't respond well to tinkering.
Regarding the skill thing. Why should class A have more skills than class B? It all boils down to siloing and the game forcing teamwork on the players. When one class can't do everything, as was the case pre 3e, then you need to have a mix of classes so that when the problems crop up you have some one who can deal with them. Traditionally thieves are the skill monkeys it was their job to scout, find traps, locks, look outs and deal with them then get back to the party and let them know whats up. In pre 3e editions this meant having special thief only abilities but when 3e added the skill system things which had been thief only were then pushed over to the skill system that all classes had access to. But to be able to do the things they used to be able to do and still be better than other classes they needed to have a ton of additional skill points.
And your right any character should be able to do those things, but the rogue and bard should be better at them.