As a DM, did you use the Keen Hearing/Smell traits? - Single Choice
Did your players (or you as player or DM) abuse those traits? - Single Choice
Will you miss those traits moving forward? - Single Choice
Should WotC issue errata & add them back for the MM versions? - Single Choice
Honestly, having smell be a special sense (non-targeting, like tremorsense) would probably handle that case better than advantages on smell tests, and it was never clear when to apply advantage on vision/hearing, since an awful lot of tests are both.
Okay, and how often in play have you had guard dogs rolling for perception in a circumstance where they wouldn’t be using their ears and noses?
As a DM, I allow players to be creative, and this has happened more than once. Using strong smells and distracting noise are things you can easily do in D&D, and smart players should use these when sneaking into a castile or dungeon. (In the most recent case, which was 2 years ago the players were stealing something from a Castile and ran a full distraction event on the Guards and their dogs. The players pulled it off masterfully btw. )
I'm not following your point at all here. All that seems likely to do in 5e is cause disadvantage, the same as it would now, with approximately the same net effect. And "more than once" is not the same as "frequently"- the vast majority of the time an NPC is making a Perception check, it's going to be because they're searching for one or more PCs, at which point all applicable senses will be employed. Ergo, rather than make the bonus a separate effect the DM needs to remember to apply, it's more efficient to just boost Perception.
I am agnostic to how they have historically applied mechanics to represent superior sense of smell or hearing as they have historically done so poorly. I just liked it when you were reminded in the stat block to factor in that a particular creature had superior senses. Agree the most common application is scenarios where all senses are relevant but the small Perception bonus is a paltry representation of animal sensory superiority over humans.
In general this change is moving in the wrong direction as the Expertise bonus given low level Beasts (+2 in all cases I've seen so far) is significantly inferior to the static +5 formerly applied to Passive Perception representing advantage. You could argue that this ability was moved from situationally applicable to generally applicable but as noted above - in almost all of the most common scenarios these abilities would be applicable.
Pragmatically all senses should work like Vision - i.e. creatures are obvious to you unless otherwise obscured from that specific sense. A simplification would be "Keen Smell/Hearing - You are aware of and cannot be surprised by creatures you can smell/hear. Likewise, you do not suffer any of the effects of visual obscurement if you can smell/hear a target."