They'd win quite few more if they fought for open drivers and firmware from the big names as well. There's a lot of people trying to make that a thing, but support from Dell could really push things forward.
Not even just 3rd party chipsets: I bought a first generation linux-based developer XPS13 and a proprietary Dell chip that only served to check that the power supply was an original Dell malfunctioned, so my laptop refused to charge and would limit CPU speed. I was left with a useless slow 13" desktop.
AFAIK the tg3 driver exists because Dell fought Broadcom for it, although that was years ago. Probably even Dell isn't big enough to budge NVidia.
But AMD maybe? I think they're big enough that if they started doing it, NVidia might consider doing at least a branch of products that had open drivers.
AMD already has open drivers. I suppose Dell could apply more pressure by shipping laptops with AMD GPUs instead of NVidia.