First, as mentioned, all pools become saltwater pools in time. The salt is a byproduct of chlorine, acid and humans. It does not evaporate. It collects in your water. If something is placed near your pool that will react to saltwater, then that's going to happen whether you enjoy the incredible convenience of an SWG or not. Maybe slightly more slowly in a non-SWG pool, but that's about it.
Secondly, UV systems do sanitize water, but here's the rub. Chlorine is both a sanitizer and an oxidizer. Once it's added and mixed in, it protects every cubic inch of your water, almost instantly. It's killing germs and burning up organic matter, 24/7, everywhere in your pool at the same time, whether your pump is running or not. UV systems work by passing water past a component that subjects that water to UV. Think about that. It's only sanitizing the water that is passing through it, and only when the pump is running! It doesn't treat the water out in the pool, and doesn't protect it out there either. There is no residual effect after the water is treated.
So here's the gross part. If someone pees in a UV-only pool, or a bird or other critter uses your pool for a toilet, or someone brings a pathogen into your water, that ickiness distributes and floats around until it happens to get sucked up in the skimmer and run past the UV. How long do you think that would take? With chlorine, remember it's everywhere in your pool, the ickies get attacked instantly, and are made harmless and then get burned up in short order. Which pool would you rather swim in? That's why your builder no doubt also included some sort of chlorine dispenser, because he knows, or should know, that UV alone does not provide the residual protection people need, so chlorine is added. Huh? Yah, a UV pool has to be a chlorine pool to be safe enough for people! They'll tell you that you can use less chlorine, and that's probably "not quite a lie." But if you follow TFP guidelines for proper chlorine levels, you won't "feel" any chlorine. So all you need is chlorine. An SWG pool and a non-SWG (liquid chlorine only) pool sanitize and protect the water in the exact same way. An SWG just makes it a whole lot easier: it maintains a constant level of protection without you having to buy, lug and pour in chlorine, every day. And both cost about the same (some say an SWG is less expensive over time). Which pool would you rather own?
And unless you're building your pool inside or under constant shade, you already have UV protection. It's called the sun!
Say "no" to the UV and decide if you want to have the PB put in an SWG or add one later, maybe DIY and save some money. He'll likely then suggest a chlorine tab feeder, if he hasn't already. But you cannot sustain the use of tabs. They will eventually backfire and cause you problems that can only be resolved by draining water out of your pool and replacing it with new water. You really only have two choices: buy, lug and add liquid chlorine on a regular basis or use an SWG.