There are a fair number of i550's being built currently. But more to the OP's question of why are the home build thread counts down? I think it a cyclic phenomenon of Sport Boats being the new (10 years ago) exciting thing and ambitious home builders not wanting to wait for the same old, same old
designs. Many good new boats were built. Some became production boats.
The cost of a used boat is fractions of new so some turnover is normal in the first couple years. Then turnover rate increases as people leave classes for any number of reasons. The number of boats available satisfies demand. Every design goes through the cycle.
There exists a small group of individuals with enough courage and persistence to initiate building a boat for whatever reason satisfies them most. Sometimes it is lack of money or remote location which drives the builder. There are i550's in Irkutsk, on Lake Baikal. a group in Croatia, a group in Hobart and a group in Portlandia, Oregon fer' gosh sakes (someone should tell them to come in out of the rain). They do seem happy, though.
So the little i550 has found a niche. Its a simple three panel hull design that has achievable tolerances in the measurements for home builders. Rigs can be any material you choose. Deck layout is open to interpretation except for one measurement. Keel and rudder are open to "artistic interpretation" except draft and chord length of keel. Simple.
So there are boats out there being built. i550's among them. Watch for foilers coming out of garages, but I think it will be still in the medium future. The jump in foil building skill is probably the greatest hurdle.
i550 Facebook and i550class.org