Are we stuck with what is here now? or if 10 years down the road serious changes need to be made to the system; can that be done with a decentralized system? How would you make changes to a decentralized system like this?
The power to change the Bitcoin protocol depends on the willingness of the economic majority to adopt the changes.
A wishlist exists of some changes that have been considered.
If the change is one that devalues the currency, then those who hold coins or plan to hold them in the future will be less willing to adopt the change(s). Some suggestions have been discussed and rejected so many times that a separate list of prohibited change suggestions has been compiled. If the change is one that improves Bitcoin without devaluing the currency or harming it some other way, those changes could be adopted. If the change is not really necessary it might not be adopted, especially if there is the possibility for unintended consequences to be the result.
A decentralized system can be changed, yes, if the majority of users using the system start using a changed client or the changed client is backward-compatible. The Bitcoin 0.8 upgrade caused some problems, as not everybody upgraded and 0.8 had a bug in which everybody needed to upgrade.