I struggle with this topic. I have 2 STI 40S&W 2011's and have been think of taking one and getting a new slide and barrel to convert it to 9mm. I have 7 9mm 1911 DS/2011 mags (5 SA Duramags and 2 Atlas) that I use with my Prodigy. If I do the conversion I want the slide-barrel FIT to my frame so I can switch back and forth from 9 to 40 and back at will.
Problem is, as others have said, the Brass. I only have about 4+ thousand pieces of 40S&W where currently I have about 9+ thousand 9mm once fired pieces and I'm still collecting.
40 brass is hard to come by and expensive to buy. 20 years ago it was easy to pick up. Now If I get 20-50 pieces every now and then that is a lot.
As to optics I'm not interested. I still shoot iron sights and will continue to do so forever. I can still see the front sight fine. As for a DOT to see that clearly I need to wear my low powered reading glasses and then I can't see the target clearly (I've just completed my seventy second year).
Oh and I do not need or have a Flash Light attached to any of my guns and I never will. But that is a topic for some other thread.