There is a big difference between niche and need. There is a niche between the 40S&W and the 45 asp, but do we need the .43NAB (newer and better)? No, we don't, but it wouldn't surprise me to see it made so a company can attach their name to it and it wouldn't surprise me that the FBI would have to have it.
I didn't say the 40 was a bad caliber, I said it wouldn't do anything the calibers that bracket it wouldn't do. The just isn't that much difference in performance to justify a "need" for it. Notice I said need, I didn't say it didn't justify a desire for it. If you pick it as the caliber you want that is fine, it will do a great job for you but it won't do it better than a 9 or 45 will do. However given identical guns, one in 9 and one in 40 a shooter will be just a bit faster with the 9. You cannot change the laws of physics with training, the lower recoil impulse will mean faster time back on sights and target.
9 mm Luger - Recoil Impulse of .78 ms- Recoil Velocity of 17.55 ft/s (5.3 m/s) - Recoil Energy, 6.84 ft·lbf (9.3 J)
40 S&W - Recoil Impulse of .88 ms- Recoil Velocity of 19.73 ft/s (6.0 m/s) - Recoil Energy, 8.64 ft·lbf (11.7 J)
There is a solid reason competition shooters like the 9 for speed competitions, they can run the course of fire faster. Again the 40 is a good round, just not necessarily an improvement. It does give you another caliber choice when picking a firearm.
As a side note...the 38 Super+P will out perform the 40S&W in energy delivered to target and in penetration. It will also work in the 9mm platform. But it is an old round and the FBI has always had a history for going with the newest thing available.
I do have proper training and know proper technique and I am faster with a 9 than I am with the 40 and faster with a 40 than I am with a 45. While I don't shoot as much now as I did when I was younger (I'll be fifty next year) I was a very active shooter in several disciplines. I still win the occasional IDPA match and am an certified Instructor for both LE and civilians. I have owned 40s in the past but just didn't find a need for them. I tried several guns besides the USP that detonated in my hand but the knowledge I was faster with the 9mm always drew me back to it.