Mongoose is still quite formidable, especially against Control, Fishesque, and decks with low threat counts. Besides, It might be a matter of semantics, but if you drop Mongoose, at least in my opinion, you are no longer playing Thresh, but UGx Fish.
IMO this is threshold due to semantics and heritage of the deck name, but no longer qualifies as that. Perhaps if you include the lonely mystic enforcer, but even with that, you only have ponder+brainstorm as cantrip to reach some kind of threshold for your mongoose. On the other hand, the deck is not defined by this card...if it where so, we would still be using werebears instead of goyfs, don't you think so?

Talking about the critter, don't get me wrong, mongoose has won me so many matches that I really like it, but not in a deck that is not getting the seven cards in the graveyard quick enough. If you run builds that feature wasteland, ponder, daze, lightning bolt, etc, threshold is so quick that mongoose is a monster.

With the build we are discussing, wich I see as CB dependant (aren't we playing 4x MD?), you aren't relying too much on graveyard, but on CB to stablish control. Other than that, you add 4x 1cc more to a deck that has to have diverse cc for CB to be relevant.

I really think that threshold (or NLB, or Bant'o Goyf, or NLBant, whatever), has to evolve and leave older elements that no longer belong to a meta or archetype. You no longer use stifle, predict (well sometimes), and so on, so leave space for new and defining cards. Better have somthing new (RWM, TPredator) adapted to the new meta, than something old as mongoose on a deck that no longer needs it.

I know it's meta dependant, but mine has Goyfslight all over the place as well as Gobos and Merfolk, wich are omnipresent. I'm matching so many vials and quick rushes that the addition of jitte perhaps do the thing for me. With the tutor, you get a virtual second copy of the artifact, as for the much needed EE on other MU (ichorid, merfolk, gobos), or pithing naming factory against landstill. I'm about to test as a 1x MD, and see if it works fine in my meta, because it seems quite techy and unexpected.

My last point on discussion is the counter-suite:

4 CB
4 FoW
3 Daze
2 Counterspell

My reasoning for this is quite simple. As games go long, the use for daze is to be pitched for Fow or as a cc2 for CB. Sometimes, all you need is the sheer power of hard counter, and in those times a counterspell is on top with the library, so you can use it to CB as daze, and to counter that tombstalker, fireblast or similars that can only be answered with Fow. As the meta evolve, you can't no longer rely on your 1,2,3 cc for CB to completely lock the opp, and people start to mess with cc in order to break the balancing (ever played EE for 2 paying 3 mana?) These are the cases you want to see the Counterspell waiting for you.

On the other hand, people will assume you're playing 4x, and it also creates the illusion of "always dazing your spells".