Then you would presumably still be able to use the remaining USB-C ports to charge it, with the MagSafe merely an option. That would not get many complaints. If they violated that they would deserve the complaints.
The thought experiment is intended to be the same as reality, just in the opposite direction.
But if they add magsafe while also having USB-C charging then it's much better than either option on its own.
And keeping USB-C charging does not require extra ports, because you need the USB-C ports anyway to do USB stuff.
But you were using it to make an argument that the complaints were invalid.
So I'm going to reply to that specific argument, by pointing out that when Apple changes the IO ports to USB-C and enables USB charging, it does not require them to remove the magsafe port.
In other words, apple went X -> Y. But adding Y reused IO ports and could have coexisted with X. Even though Y might be better than X, it was a false dichotomy in the first place.
It also entrenches the old standard rather than making way for the new one. If you have a laptop with both USB-C and MagSafe, and you need a second charger, you might buy a MagSafe charger if it's cheaper or to use with an older Macbook. Whereas without MagSafe, you'll buy a USB-C charger — the new technology.
If you want to do a transition, you have to go all in. Having both the old and the new port merely delays the transition, causing more pain in the long-term. Get it over with.
Thought experiment: say Macs had USB-C for 10 years, and the latest Macbooks switched to MagSafe. There'd be a lot of complaints about Apple locking us in to their walled garden, and how we have to buy a proprietary overpriced charger now, which can't charge other devices, moreover.
One technique I try to follow is an anti-knee-jerk reaction: if everyone has a knee-jerk reaction about something, I remind myself of the advantages of the alternative.
Ultimately, the only true test is time. Wait 2 years and let the emotions cool, and you'll know if the outcry was justified.