If you have lightning headphones, then you've got to have an adaptor to use it with any other device, and that means adaptors you're regularly taking on and off. You've then got to get another adaptor to charge the phone at the same time. Lightning jacks are also easily broken, compared to 3.5mm jacks that can usually swivel.
If you have 3.5mm headphones and need an adapter, then thats an easily breakable adaptor, easily lost, easily snapped and expensive. Then you also need another adapter to charge as well.
If you have l bluetooth headphones then thats another battery to charge, more drain on the internal battery of using bluetooth, connectivity issues, the headphones themselves are expensive and connecting bluetooth headphones is far more annoying (tuning bluetooth on, waiting for a connection) than just plugging in a jack.
At the current time, we already have the ideal solution - if you want lightning headphones, you can have them. If you want bluetooth headphones, you can have them, if you want 3.5mm jack headphones you can have them.
Removing the 3.5mm jack removes connivence at just about every turn. It is not an environmentally friendly move (creation of unnecessary adaptors, more battery consumption an dumping of 3.5mm devices) and it serves probably purely as a money maker to Apple (ca-ching expensive adaptors, proprietary devices).
I absolutely detest the idea of a proprietary headphone jack... 3.5mm jacks are one of the few triumphs in the computer world of standardisation. If the entire computing industry was moving towards USB-C headphones (which I don't see happening just yet) I'd say ok, replace the 3.5mm headphone port with a USB-C port and also have the lighting port but thats never gonna happen. I hate the fact that Apple has so much power in the industry. Literally the only reason other manufacturers will remove headphone ports (besides ultra thinness) is because Apple is doing it - not because it is a good move, but simply because Apple is doing it so it must be trendy.