While we are speculating...
You would think @Menards at least entertained purchasing portions of the Rail King line such as passenger cars or even some engine tooling. I could see them focusing on only a few engine models and produce those in relatively high numbers. IE., they may only produce Rail King F3s and baby hudsons, but we would get them in different road names throughout the year.
What about engine warranty / service work? They will just replace the whole unit . No different than when you return a faulty drill. Or they could contract with repair shops to perform the work on their behalf. Menards doesn't have to do the work themselves, just be able to fund it through proper warranty management.
Engines aren't Menards M.O.! Not to date, but it's not a stretch either. Toyota gained its foothold in the 70s and 80s with cheap, yet reliable vehicles. Then in the 90s, moved into the higher-end market with Lexus. Menards has a foothold in traditional rolling stock, it certainly could take advantage of producing a few popular engine models which have a higher margin than $20 boxcars.
What about operating system/could Menards have DCS in their engines? From what I saw, Atlas was "licensing" DCS, so Menards could theoretically do the same thing if they wanted. On the other hand, they could produce low-cost conventional engines that are eligible for upgrades (similar to Williams). They could also contract with somebody like BlueRail to provide DCC electronics in their engines - that could be very interesting. IF they went full battery power, with two rail O gauge track - that would be a disruption to the market for sure.
Bottom line, if Menards wanted to pursue engines at some point in their "Train Stuff" existence, Rail King would certainly be a good opportunity for them. In the end, the total cost and projected ROI informed their decision. Although, as a private company, if the CEO/President/Owner wants to do it, they are going to do it.
Speculating is fun...