If only. I have several lenses for a variety of situations. A lighting rig for indoor shoots which includes bounces, tripods, umbrellas, cords and rigging. Then there's the tripod, plus a back pack for the smaller stuff. Not to mention backdrop equipment.
There's a big difference between casual hobby and serious hobby.
Consider, nobody says you need an electron microscope to get serious about photography, but they can take some amazing photos even without using photons...
Even professional photographers that only do a specific kind of photography professional nearly always have amateur interests in other photographic subjects that require different gear.
I have even known someone to go the other way and sell off there kit for a point and shoot. They where into candid photos so always having a camera was more important than fiddling with a massive beast.
Granted, camera's are cheap to most of the HN crowd, even blowing 30k over 20 years is just not a big deal. But, just because gear is 'cheap' and 'better' does not make it necessary.
A single lens camera is as limiting as only owning one size of screwdriver, or one knife.