Its been said before, but hopefully I can say it more convincingly
As monster design evolved through the years, developers began to realize a fundamental truth....a fight with 4 weak monsters works differently than a fight with 1 big monster.
Further, they realized that the 1 big monster fights were often worse than the 4 monster fights....that one strong monster was simply not pulling the equivalent weight.
The reason is this...the rules system is designed with those standard monsters in mind. The one set of actions a round, how defenses work, how effective debilitating conditions are, etc.....everything is designed around the party mentality. Simply scaling those numbers doesn't fundamentally fix the problem, because so many of the rules remain the same.
One of 4e's greatest contributions to the game was this recognition, and it provided an avenue to alter game design through the elite and solo tags.
The solo tag gives license to bend or break the normal rules. A solo monster gets to act more often, repel effects that a normal monster could not, do more area damage, and be significantly tougher than an equivalent single monster.
And even here it took 4e a long time to develop solo monsters that really did the job. The early attempts were pretty bad to be honest, but eventually I think they got it right.
4e got a lot of stuff wrong, but its approach to monsters was spot on. Those rule exceptions created better encounters and more balanced and yet exciting fights.
I think there is a place for boss monsters in the game, but I don't think designations such as elite or solo belong on goblins.
I don't agree with this argument myself, but its an argument I can very much understand.
A lot of what we are debating isn't so much mechanics as it is the flavor those mechanics invoke.
I think we can all get behind the idea that dragons and beholders are just different from your usual monster. They are special, powerful, the exception to the rule. So for them to use fundamentally different mechanics is not a huge flavor issue imo.
However, the same cannot be said for the "elite goblin". For some, it can be a flavor break that one goblin has X abilities, but the elite goblin has all sorts of weird different abilities other goblins don't.
So perhaps for the humanoid monsters, working the tougher monster angle is the way to go for boss encounters. But for other monsters, I think the solo and elite tags are extraordinarily useful and should be maintained.