That's true. However, I'm sticking to my Corporate Champion theory. They [Chrysler & car companies in general] could have (and often do) ship all that 4th gen tooling down to Mexico, Brazil, Chile, or Australia and continue producing legacy products in other markets. And tooling doesn't last forever. Stamping dies, molds, etc have a lifespan which need to be replaced.
Anyway, by getting rid of the 4th gen a very strong argument is (and has always been the case ...so we know that's how it usually goes) would be that by streamlining parts & costs ...expanding all models under the 5th gen model family. It would reduce workforce, combine supply-chain componentry, reduce plant costs, maintenance of old tooling, robots, etc, put all production under a single umbrella. That would reduce overall 5th gen production costs, help pay back product development [which includes tooling] sooner (i.e. making more models using the same 90% basic parts & tooling). It would also allow them to show higher new-product sales and give a boost to overall 5th gen profit numbers ...on paper (which is everything when releasing corporate figures to investors, every quarter).
Because it's such an odd and rare decision, there's got to be a high-up corporate Senior-VP who, for whatever reason, likes and/or wants to see the 4th gen Rams continue. At the marketing level you can create a argument just about any way using whatever production/cost/profit numbers you cherry-pick to justify a case. They do it all the time at the top levels. That's how you get your decision through. In absence of that, the case could be made tomorrow that it costs too much and eats into our 5th gen profits by continuing to produce two of the same basic products. But someone is saying, "Now hold on ...we're also capturing marketshare by ....blah-blah-blah". And that argument seems to be winning.
Or possibly, 4th gen production has just been a place-holder all along, until the compact pick-up can be produced at the same plant. But due to Covid and supply chain issues, the can has been kicked down the road longer than even Chrysler planned. But due to steady sales, they haven't tinkered with the formula (yet).