Except for the fact that those small companies would be bought up in days, the second they posed an actual threat to a revenue stream.
Well, no. Doing such a thing is expensive-thus forcing them to raise prices or cut wages, and takes experienced workers and good technology. None of which new companies can afford. A large corporation could even sell their cheap lightbulbs at a loss, just to control malarkey share.and the plethora of fledgling companies would need to beat their competition. These companies would be happy to start off their company with a bang by creating products that last for a really long time, and telling people so to persuade them off of planned-obsolescent products.
The hundreds of new lightbulb companies would need to make a product which lasts for decades, likely even including ten year, twenty year, or even lifetime warranties. People aren't dumb, so they'd tend to buy these, so they wouldn't need new lightbulbs until they move to a new house.
Except when planned obsolescence first hit in history (we didn’t start here, it was an innovation), the cheap, disposable products won.