> Furthermore, retrofitting a program using synchronous execution to use asynchronous execution is a tremendous amount of work as you need to do it bottom up to

> Furthermore, retrofitting a program using synchronous execution to use asynchronous execution is a tremendous amount of work as you need to do it bottom up to maintain compatibility during the transition. Can attest to that. I was part of the team that rewrote Firefox to be fully async. Took us years and we could not maintain compatibility with XUL add-ons (async was not the only reason for this, but that's where the writing on the wall started to become visible).