If the focusing drains energy from the mind, it's the wrong kind of focus. Real concentration (buddhists call it samadhi) doesn't come from forcing your other parts of mind to do what "you" (the small, narrative mind-process) want, but rather to unify the whole mind around single intention. This state is not only stable, it's more efficient, because there's just one thing to do rather than tens of conflicting intentions.
It's more efficient and calm than Default Network Mode, which is basically a garbage can for the mind.
You don't even have to get into buddhism, there's a lot that science already knows about eg. the flow state, which is a mini-samadhi. Also, Judson Brewer discovered that DNM is not fundamentally 'default', as it doesn't activate for long-term meditation practitioners. Such people (some of them really high functioning) report that great thoughts just come to them out of nowhere. Clinging to the thought forcefully only reinforces the illusion that 'we' are thinking it, which is not true and counterproductive, as it obstructs the dynamics of the mind. Just let the mind figure the problem out and let you know when it's done. DMN is an impostor that pretends that everything done by the various diverse mind processes is done by the narrative fiction centered around the feeling of 'I'. It's not, it's highly distributed, there's no driver and no center.
The more we think otherwise, the more force we have to put to make the mind obedient.