Well this is Hacker's news, you label something legacy like code, its never a nice thing or showing some respect to product/creators, rather contrary. TBH its also the first time hearing about this term, its simply not common, not even here and much less in general population.
And I have to strongly agree with OP, I couldn't care less about fate of FAANGs of these days, but I do care about those 'legacy' businesses tremendously.
As for original topic - if it works for 3-4 decades, don't be the stupid guy and change it. Tremendous risk to core business with little to gain.
Your comment doesn't make sense, why in Hacker News should I show more respect to Walmart and JP Morgan rather than google or apple by calling the former legacy industry and the later big tech, and why are you so worked up about it?
Relying on the existence of one vendor with highly unportable and unmaintainable code carries its own risk and my post is asking whether it justifies the cost.
It also gives you the opportunity to use their mainframes advantages to the very max. Not the "common feature set" but the very "best features" you can get out of your tech choice.
I do the same, I have run PosgreSQL for more than 2 decades now and I don't care about portability to any other database, all of which I consider inferior (and yes, I do follow most of their releases).
And I have to strongly agree with OP, I couldn't care less about fate of FAANGs of these days, but I do care about those 'legacy' businesses tremendously.
As for original topic - if it works for 3-4 decades, don't be the stupid guy and change it. Tremendous risk to core business with little to gain.