Okay what board are you buying Chapel on that you don't want to transition to a "pure copperless" deck? That's every Chapel board. That's the whole point of Chapel.
Not so. There are definitely situations where it's correct to keep some Coppers around with Chapel and even more situations where it's correct to defer trashing some Coppers. Sometimes you need the Coppers to reach a high price point and the tempo loss from trashing and adding additional economy would be costly.
Example 1:
Tournament. With Chapel + Tournament, it's super important to be the first person in the matchup to buy a Province. Potentially, you can block the opponent's Tournaments (and subsequent Province buy) and snowball into an easy victory. So, it will probably be correct to keep 2-3 Copper to afford your first Province (even if this increases your stop card count above 4).
Example 2:
Pathfinding. For an extreme example, add
Magpie. There are situations where keeping some extra Coppers around to buy Pathfinding will not compromise your ability to draw your deck, but will enhance your economy.
That being said, it's (almost?) always correct to trash as many Copper as possible with Chapel the first time you use Chapel. I can't think of an example where it makes sense to buy a $5 over trashing 3 Coppers with S/C/C/C/Chapel on turn 3 or 4.