Is eMMC really a good choice for a low-volume long-product-life design?
I think there is just not as much demand for emmc.  We looked at it and decided it made more sense to just roll the controller into our chip.  We were attracted by the integrated controller at first but that feature adds cost to raw NAND so it didn't make sense for us.  I bet a lot of other people feel the same way.

I would think using sd cards for long term deployment would be risky especially now that they're using TLC flash in them.  Talk to your ae about bit rot,  they wouldn't give exact numbers last time we spoke but they weren't pretty less than a year if I remember right.

If you are concerned about supply in the future I'd encourage you to contact your micron rep.  They have a separate list of memory and flash parts that they guarantee to carry for a long time, and if memory serves me a smaller list that they'll carry for even longer :)