I'm not sure why you fear custom water cooling, as unless you drive an old VW or Porsche any car you have is likely to be watercooled, and its engine costs more to replace than a 7900x system too. If you really wanted to you could even use all AN fittings instead of barbs and clamps.
Porsche had to move to watercooling in order to avoid detonation at high boost pressures, as air cooling was unable to remove the extra heat quickly enough. As stated before, big air works as well as AIO if you have the space. Water cooling allows you to remotely mount the radiator to avoid introducing waste heat into the case, but the hoses that come with AIO are usually too short to move it far. I run an old truck A/C condenser underneath my house as the radiator. There's no fan noise or heating of the room because, well it's all outdoors. And as the lowest point of the loop I never need to crawl out there to bleed air from the system.
BTW the GPUs probably benefit more from water cooling as there's so much less room for a big air heatsink on those that they can get quite noisy under full load, plus the TDP of each can dwarf the 7900x's puny 140w. As for the 112w 7740x, it's fine if you only want 4 cores, and to be only able to use half of the RAM slots of your motherboard. Yep, it only has a dual-channel memory controller because it's just consumer Skylake, only on LGA2066. And you can answer how many x16 cards for that 7740x easily because just like 8700k, the chip only has 16 PCIe 3.0 lanes. 7820x has 28 and 7900x 44. Intel loves their levers and switches for product segmentation, so if you want more, you must pay a lot more.