Lol i didn't either until recently when a friend showed me she was using both xD
I'll respond to you sections of what you wrote
'The players who chose to get geared'- a small section of the AL endgame community were already geared with 6/6 mire set even before anybody had a whiff of something like the introduction of the swamp temple challenge. So no, not everyone got geared specifically for this event by buying set items. Players have been geared with full mire set before this existence of this event, it's only that when the event was announced it gave them automatic advantage over other players.
'Many players spent a lot of gold and lot of time to build up strong toons'- Yes, but does not guarantee that every one of that player who built a strong toon relies on set items which comes handy for only certain specific maps- in this case the festerfang expansion. Players build strong toons by relying on other better gears too (excluding the mire set items- example new arcanes).
'brilliant event to challenge any endgame player for same class lb spot'- this I find hilarious tbh. How many players really do have the full sets? Not many do. I'll talk about this from a rogue lb point of few since I play rogue class. For instance the full mire sets give a buff of 8-10k damage, 11-12k dps (I don't know what is maximum stat it can give, so correct me if I'm wrong), you really think someone with a 3/3 buff or someone who built a strong toon with arcane weapon, arti, mire 3/3, arcane ring and amulet can actually compete with someone who has a full set buff (plus damage buffs)? Players who have the advantage of a full set will be competing amongst themselves. It DOESN'T and CANNOT allow for competition among different strata of players. The ones with full sets will outdo the ones who have the partial set, the ones with the partial set will outdo those who don't have set or good gears and so on.
'Advantage came from effort and it shouldn't be touched'- endgame players have made effort in different areas. Some invest a lot in very high Ms gear for timed lb, some in very high boss damage (again for lb purposes mostly), some in extremely good PvP gears for pvp purposes, and set items (before the introduction of this event) to be more efficient in raid maps or the maps for which set items have been released for. For me having the advantage of such sets is appealing to the extent of having their utility for the purpose they were introduced for. Nothing more, nothing less. Hypothetically speaking, if other events are introduced which favour only Ms, or only boss damage, etc etc. when not everyone invests in the exact same thing would make it skewed for the ones who don't have them.
From that point of view, it makes a challenge less of a challenge for some as with specific gears, pets and strategy you can breeze through this lb you mention, and this my friend is a very tiny section of endgame players. Hence I'll stick to what I said before- not everyone has an equal footing, and as this is an event for all endgame players a certain kind of parity must be maintained.
All the best for the event :)
Where is the goblin event prewiev? :d