I believe everyone, including Colt, is incorporating MIM components, at least in their entry-level products. Higher-end Colt and SA guns likely won't.
And just as a FYI, all Sig's I'm aware of are Series 80 guns, and only in the modern 1911 style. Personally, I have no problem with Series 80 guns. I've got one on my hip right now. I do have a preference for Series 70 guns, though.
If you're interested in a more modern 1911 (i.e., beavertail grip safety, extended thumb safety, etc.), then Dan Wesson is probably one of the best values out there. Several different styles of full-size 1911s available, well-made and well-fit, good triggers, Series 70 firing pin, and no MIM components. Before pandemic availability became an issue, they'd probably be in the $1200-1500 neighborhood. Don't know what they're at now.
If you're looking for a basic GI-style 1911, of the makers you listed Colt, SA, and RIA make them in that style. Colt would be at the upper end, RIA at the lower end, and SA in the middle. Personally, the Colt Classic would be my first choice, but I'm a little biased towards Colt. Series 70 firing pin, but with some modern touches (solid bushing, bigger sights, and lowered ejection port) that, I think, are an improvement while keeping the traditional aesthetic. But I've also been looking at the SA Mil-Spec Defend Our Legacy (I think that's what it's called) model, which also has some nice features while still approximating the GI-style. Don't know much about the RIA models, but from all accounts I've read they seem to be well-made guns and a very good value.
A couple of other, more modern options...
I really like the Colt Wiley Clapp models. I believe the MSRP for the Govt Model version is $1299.
I also really like what I've seen with SA's Ronin Operator 1911s. I want to say the MSRP was somewhere in the $800-900 range. Lots of good features. It uses a blued slide with a stainless steel frame. I've always been a sucker for two-toned 1911s.
FWIW, I'm not too concerned about MIM parts. They've been in use for several years with several makers and I can't recall reading about any of them failing. But the nice thing about 1911s is that it's not too difficult to replace MIM parts with bar stock if that's what you want to do.
Fortunately, there are a lot of 1911s out there so you should be able to find something that balances your needs/wants and your budget. Of course, with the gun market situation the way it is now, it may be difficult to find what you're looking for.
Just my opinion. Hope that helps.
