As an example, a dev with say 4 years of experience overall that happens to be working at a bank who might have had some solid internship and side projects upon graduation won't be 'locked out' to the same degree as someone with 15 years at that same bank. Someone with 25 years at that bank who worked on very similar projects may be seen as unemployable by most startups.
That junior dev might not have any kind of preference or necessity for process, but just wound up at the bank after graduation. It's these devs that probably suffer.
The person who stayed 25 years (ageism aside) probably did so for a reason, and some will assume that the reason was process and predictability in their work. Someone that stayed for that long and didn't get bored is probably going to be passed over by most startups.