What if developers create an architecture?

Product Owner goes to Devs and asks to add a cool feature in 1-2 sprints.
Devs think hard and find the way.
PO is happy - what a great velocity!
Devs are happy - we're very cool!

6 months passed.
Product Owner is very sad - these s̵t̵u̵p̵i̵d̵ strange developers take months to add a small feature now, they break existing features constantly.
Devs are sad - in order to add a small change we have to either rewrite the project for months or add new workarounds that will be broken in weeks.

How has this happened?
System architecture was developed with a tunnel vision paying attention to short term goals of 1-2 sprints.
The fate of such systems is always the same - a big ball of mud.

How can an architect help?
An architect thinks strategically, sees a whole picture.
He/she doesn't care about short term goals of sprints.
An architect sees how broken bicycle chain can lead to a broken leg in 2 months.

Does it mean we should not delegate architecture creation to Devs?
No-no. This is a pretty good approach, very popular in Agile world.
But we should attach an architect sometimes/always to guide architecture development.