""QSL" at the end mean?"
It takes the place of "How copy? break"--HW? BK
Except that we CW ops don't do that stuff in a contest.
Here's a typical Field Day QSO on 'phone. W1ABC's transmissions are in parentheses, K2DEF's are in brackets:
(Hello CQ, CQ Field Day, CQ Field Day, here is W1ABC, Whiskey One Alpha Bravo Charlie, W1ABC, Field Day)
[Whiskey One Alpha Bravo Charlie, here is K2DEF, Kilo Two Delta Echo Foxtrot, K2DEF]
(K2DEF this is W1ABC, please copy five alpha, five alpha, eastern Mass, eastern Massachusetts, QSL?)
[W1ABC this is K2DEF, roger, QSL your five alpha eastern Massachusetts, here is three alpha, three alpha western New York, three alpha western New York, QSL?]
(K2DEF, QSL, thank you for the contact, 73, this is W1ABC, QRZed Field Day, QRZed Field Day from W1ABC...)
And that's a short one, some of them drag it out a lot more.
The same QSO on CW:
(CQ FD FD DE W1ABC W1ABC FD)
(K2DEF TU 5A 5A EMA EMA)
[3A 3A WNY WNY W1ABC DE K2DEF]
(R TU FD FD DE W1ABC W1ABC FD)
Same info but not a wasted dit.
Note how the CW sending op doesn't ask if the other op copied the info, nor do the ops repeat what they copied. A simple "R" (meaning "received") does the whole job. Often the the "R" isn't needed, because it's obvious by the response that the info was copied. (K2DEF would not send his info if he hadn't copied W1ABC).
I don't know why 'phone ops drag it out so much, particularly on Field Day.
It can be that ops who started out with CW only developed a pattern of doing something and associating a Q code with it.
But that's rarely the case.
At least I don't hear hams saying "kay" at the end of a transmission...
The migration of Q-code to modes other than CW makes perfect sense to me.
It makes sense to me only if the use of a Q-code or other CW abbreviation actually saves time or clarifies things.
For example, as you point out, "73" has a meaning all its own regardless of mode, and is certainly faster and clearer than "best regards".
But often that's not the case.
73 de Jim, N2EY