We had an issue where a toilet that worked fine since we moved in 6 years ago recently would not flush. I took a plunger to it and snaked from the roof and it helped in that the toilet would slowly drain out (15 seconds) but never give that typical "glug glug glug" one gets from a full flush. I called in a plumber and a week after we last tried, it fully flushed when he tried it. It was a weak flush but it was a full flush. I asked him to snake it anyway since I was already on the hook for a service call and he opted for a 6ft augur. This further increased the flush but it's not as strong as what we are used to.
He told me that best way to fix it is to install a new toilet. He mentioned that as toilets age, the trap "gets full of stuff" "that is like cement". I had a spare toilet we ended up getting stuck with after a renovation that was never used but had been sitting outside and asked (since I had him for an hour anyway) if he could use that but said that because it had been outside, there was too much "stuff" in the trap and it could be worse than the one already in the bathroom.
I'm no plumber but I get the sense that there is nothing from normal use with a city water supply and sewage that should cause a toilet's trap to "age" to the point where it can't be cleaned out to be good as new... especially if it is not yet installed and you can access it from both ends. Should I trust this plumber's opinion?