That's exactly the approach. Seen many deals where the (remaining) founders get a big slice of new vesting options or reverse vesting shares as part of a recap or semi-distressed round.
Nothing illegal about it when the company needs the money, just one investor can write the terms they want, and the founders are on board with the plan.
I understand that, particularly in a down round, investors can push to get more. What I don't understand is what allows founders to get a side deal. It seems like that would go against fiduciary duty to common shareholders and earlier rounds.
Nothing illegal about it when the company needs the money, just one investor can write the terms they want, and the founders are on board with the plan.