I do think they will make a cheaper plastic only version in the future.
Yes, there are a lot of people who are expecting this announcement at some point. The fact that they are changing up the hardware in the Noir version to be closer to their Super NT and Mega SG models just reinforces this idea.
The NT mini is popular with collectors, but it is still very much a hardcore, enthusiast-focused device. It's price is reasonable for such a premium product, but it is not appealing to a broader audience, and out of the acceptable price range for general consumers. A solid aluminum shell is just expensive to produce, no doubt about it, and the extra video ports and cartridge slots also add to the production costs.
The Super NT and Mega SG are production-line level. They are great systems, but they are designed to be much cheaper to produce in large quantities, which also allows them to be sold for a profit at a much lower price. That's why they cost less than half what is charged for the NT mini. Gamers who wouldn't dream of dropping $500 on an NT mini might consider the $190 for the Super NT.
After the NT mini goes out of production for good, they will likely release a NES-focused 8-bit console in the same price range as the Super NT and Mega SG. It will likely have the same basic features as those systems, but meant to run NES games. It will likely have a cartridge adapter for Famicom support, like the Mega SG came with, instead of having two cartridge slots. It will ditch the built-in analog output for just an HDMI port.
Part of what makes the current NT mini so appealing for collectors is just how broad its support is, for a platform that was pretty wild in its day. The NT mini supports the Famicom Disk System. It comes with a built-in microphone port so that it can offer compatibility with the Famicom's hard-wired controller microphone. It's analog output allows for light-gun games out of the box. It's the total package for 8-bit gaming. The audience for it might be a bit more specialized, but it is definitely there.