Exactly. It's pretty inconceivable that they would have boxed themselves in from the very start at some predetermined specification. More than likely, they had a number of plans that they've either altered somewhat or moved away from entirely since they've been planning for the next Xbox.
Obviously it's to be expected they're probably well past the point now where they have a favorite design, and that favorite design is probably more or less what we know now, but I fully expect that there are enough moving parts among their existing plans that, while I'm sure not wildly different from their current plans, have some meaningful differences and are fairly advanced, well thought out designs in their own right that if they did indeed arrive at the conclusion, that a change was absolutely, no way around it necessary, they would be prepared to make that change.
After all, we are talking about a highly qualified, no doubt hard working team of professionals that are paid to get things right and account for any and all possibilities. It isn't so difficult to imagine that these folks could have from their work with AMD, devised multiple paths forward.
It isn't their job to design the most expensive possible system they can make just because they are able to, or allowed to. They have to, at all costs, find a way around having to do such a thing. Their primary goal is giving the developers what they need and designing a system that can also meet Microsoft's own desired goals. If they can do that without going for the most powerful or expensive parts, then that's what they will do, and it's certainly what they should do.
After that it's up to Microsoft to user every resource available to them to make the console as desirable as possible from a software, features and services standpoint.