It's always Lego. One brick, many bricks, one box, several boxes, one construction, many constructions, always Lego and Lego only.
Yeah, that's a commercial and a cult follower thing, not grammar.
They're legos. They are toys that are common items in many countries. Cult followers can go rabid about proper referencing by non-lego folks, but it's just a toy.
Trying to dictate grammatical rules about how people refer to the toy is similar to folks going crazy when someone says, "I'm going to Wal-Mart's." There is nothing ungrammatical about referring to the store owned by Wal-Mart as "Wal-Mart's."
A company cannot dictate the grammar of their product in the language with any authority. Million of parents refer to the lego block as a lego. In languages where pliural is made by adding an S, then they become legos by common use, which is fully understood and vivolates no rule in English, as the items are commercial, not objective language established via centuries of use.
By similar example, millions upon millions of people recognize what "Release the kraken" means, despite the native language being "Release kraken" because the suffix already includes the article attached to the noun.
Similarly, tinker toys are now common nouns as a generic reference, not requiring capitals as a proper nouns. The progression to common nouns works that way. Once enough people are doing it, it becomes like the lunar tide, unstoppable. Legos have been around long enough to be common nouns.
We don't still write R.A.D.A.R., but radar. It's convenience and use.
The maker is in fact "LEGO" but the common use is not "LEGO brick" but most sensibly "LEGO" to refer to the single item. The company will never be able to stop the process, nor should they.
But go ahead and stand in the road and try to control traffic. It's satisfying there in your chair.