I bought a new bike after getting some excellent advice from the forum. I planned to use the old bike for commuting and keep the new bike for the weekend but the truth is, as soon as I sat my backside on the new bike I quickly made up my mind that I wouldn't use anything else. I give it a good clean at night and a good spray all over before putting it back in the garage but it seems this isn't enough. The rear brake started jamming on, so I put it in for its free check and it turns out it was rusting inside. The guy fixed it no problem, but his advice was not to ride it in the winter to avoid more problems. Is this a thing or would you expect a new modern bike to be able to withstand a few weeks of winter use?
I'm clearly in the minority here but I say your bike shop owner was right. Brakes contain springs that can only be made of steel. Steel can be painted or plated. Paint doesn't work on steel, so they're plated. Plating is very susceptible to rust. Salt causes rust. The bolts are also steel (Titanium on some fancy stuff) and they too will rust, at first inside the hex hole and then spreading onwards. These brakes contain bearings and all bearing balls rust. You discovered this yourself.
The RD and FD both contain lots of steel.
The hubs and other bits are made from polished or anodised aluminium. This develops pits in the presence of salt.
Spokes have either aluminium or brass nipples. Aluminium and stainless steel, in the presence of salt, forms a nice galvanic battery that quickly cracks the nipples. Brass too, but much slower.
Headset bearings are sprayed by salt water coming directly off the front wheel. It rusts.
Lots of other bits and pieces corrode as well, paint and plating notwithstanding.
There is good reason why people have summer and winter bikes.
I guess will boil down to semantics in the end and it all depends on what you asked the bike shop guy for him so say not to ride it in winter. If you asked him how you can prevent it, his answer is correct.
No amount of stuff you spray on or cleaning can negate riding in salt water. Nothing.
Salt water spray is one of the most powerful tools to test for corrosion. Usually an item will be said to be corrosion proof to "50 hours of X-temperature salt water spray."