The reasoning is coming from people who already have kdevelop sessions on their system.
Thinking more about it, possibly more often people will launch kdevelop from the installer after first installation than on updates. And I agree for those new to KDevelop reaching directly the shell window with the welcome screen is the better experience.
If people complain because they want the session chooser dialog, we could still add the _ps desktop file as alternative launcher later :)
To 5.4 branch, please.