Yes, but some of Jill's customers come at lunch and couldn't afford to wait in the longer lines. Those customers now go to John's having waited a few days to look for an alternative. Jill gets 20% of John's clientele, but John gets more [impatient] customers than ever!

There will probably be some additional flow of customers as people realise they're too lazy to walk to Jill's/John's, Jill's was only better when they were doing the extra trade (freshness), or that Jill's is back to being quicker service (and then you have a chaotic effect as more people drift back the wait time gets longer).

The potential complexities of such simple systems are fascinating.