There is a general rule in life that things are rarely as good as you hope or as bad as you fear.
When Carroll leaves, we won't be able to replace him. He is one of the best coaches Seattle has ever had and one of the best in the NFL.
You don't replace that. Someone else will come in and for a while people, myself included, will freak out because he is not Pete Carroll. But we likely won't become a dumpster fire when he leaves either.
Regardless, Carroll drives me crazy with his ridiculously conservative bent sometimes. His being a defensive-minded coach sometimes (often) results in him not using Wilson and our offensive weapons as well as he could. But there is no doubt that Seattle will not see this kind of success repeated for a long time after he leaves...because generally, coaches do not keep winning like he has even through the rebuild.
So instead of worrying about that time, it might be a good time to really appreciate what we have now.