One of my cats has extremely long hair - lol in fact I just took a tape measure over to him - many of his hairs are 7 inches long!!!!
This summer, I did trim his hair, but only
on his underside (his belly), just from his arm pits down to the insides of his back legs & under his tail (I think this is sometimes referred to as a "sanitary clip").
With the hair on his underside trimmed short, he was clearly more comfortable laying in sunny spots by the screens, rather than finding a cool dark place to lay. I think I'll continue to do this "Belly trim" on him every summer.
He is very agreeable about having it done - I just rub his belly so he lays on his back, and I use safety scissors to trim the hair down to about 1/2 inch while he purrs away.
I would not trim his underside if it meant taking him to a groomer & having him sedated.
He doesn't mind being trimmed, and with hair as long as his, it must be easier to step out of the litter box with a little less long hair around his bum.
It's funny though - even though his underside is trimmed short, you can't even tell he has any
hair trimmed unless he flops over on his back for a belly rub.
I guess I'm lucky in the texture of his fur - even though his coat is very long, it must be silky, cuz he doesn't tangle at all. On occasion he'll get little tiny tangles near his neck under his "cheeks" but they work out really easily & quickly with my fingers.
I think for the select long haired cats who have very
long hair that are unable to groom themselves properly, the matting and pulling would be excruciatingly painful on the skin, so trimming/shaving is a kind option for them.