Does architecture maturity matter? Do I have to exercises regularly in order to succeed with a marathon?
If your level of ambition is to do large changes to your organisation, the the answer is yes to both questions. Otherwise, just get out and run regardless of wether conditions.
So what are the consequences of not preparing?
In both cases is it that you would not reach the goals, ether in the race or in your program. If you don't have any capabilities for your endeavor, you may not even survive.
In a race, the crew hopefully will take you of the track, before it's to late. In a company, the last resort is the quality organisation who to stop the program if the sponsor doesn't.
A high maturity doesn't guarantee the success of a larger programe, but a low maturity makes it impossible to reach the goals. The exception is when raising architecture maturity is part of the program.