Small boss raids are never good. Sorry, but it's the truth. They stopped making them because they got tons of backlash for it back in cataclsym when literally no raid in the entire expansion had more than 8 bosses. It doesn't work well when you're trying to go for this whole "actually making places feel like you're raiding them rather than walking into a tiny room and killing some poorly guarded generals/bad guys."
Also, small raids don't play well over long periods of time. The longer the time between patches, the more time people have to wait for the next new thing, and the fewer the bosses, the less there is to tide them over till then, progression wise.
It's true that patches are far faster to come out now, so smaller sizes could come back in that regard, but I still don't think they should. Small raids fell unepic. They feel like a waste. Look at Emerald Dream. I'm sorry, but that raid sucked. It was tiny, the bosses were a joke and the whole concept felt uninspired. Then look at both Nighthold and Antorus, both larger raids with more bosses and with arguably much better content in them.
Size matters. Small raids are meh. You want to feel like you're RAIDING a fortress, not running drunk into an nearly empty restaurant and punching the lights out of the bartender, his host and their chef in the kitchen.