In either case, it's also like cutting off your left arm because you still have your right arm.
My experience is across three different companies now, and in the first two (not my current employer who... still do it wrong but it's the best I've seen), they fired the Ops guys and expected the Systems Engineers\Software Engineers to take those roles.
No overtime pay in either company for the extra work load that was placed on them (e.g. me) of course. So the only cost the companies incurred was productivity and the quality of work. The burden was entirely carried by the staff and to a greater degree the clients who accept a shoddier product and service.
Fortunately I quit both jobs to get to where I am now, though the old timers no doubt are still working ridiculous hours and are too scared to ask them selves "can I find better else where?".
Long term, doctors aren’t nurses and patients suffer. Eventually someone dies.
This ignores that it discards specialisation, not merely quantity, and that testers are generally cheaper than developers.
And then QA does their good job and gets hate for saying that accidental nuclear genocide [or whatever] might be a bad thing so let's fix that before release to the customer.
Sure, "code to spec and you're done". But are you actually really done? Are you sure?
I was at a company that did the same (at roughly the same time) - this was a terrible mistake. But it's very hard to discuss, because any dev complaining sounds like they want to avoid testing.
Dear manager that I hope reads this:
QAs, at least the ones under discussion, aren't just mindless pressers-of-buttons. They have a skillset and domain knowledge just like (but distinct from) devs. A dev should absolutely test their own code...but they will do so with their domain knowledge which will be by definition more limited than someone for whom such knowledge is a key part of the job. Devs should test so that only problems worthy of the QA skillset make it to the QAs.
Asking devs to do QA work is asking them to build skills that are not their strength, and that will come at the cost of their strengths. It is asking them to build skills they probably don't enjoy, and that will come at the cost of their job satisfaction. Having devs do QA work is as bad as devs that test nothing and leave it all for the QAs - it's an inefficient and ineffective use of resources and time.
There is nothing wrong with encouraging Devs to deliver higher quality code to the QA/Test engs. There IS something wrong with thinking higher quality code from devs mean you don't need a QA/Test Eng.