The thing is, JRiver only officially supports Debian with MC for Linux. Ubuntu, Linux Mint, elementary OS, etc. only work out-of-the-box because those are all based on Debian. Arch Linux, Fedora, etc. only work because there's a community either providing RPM conversion scripts (in the case of Fedora) or PKGBUILDs (in the case of Arch Linux) or whatever else.
Right now, they're building MC on Debian Jessie. If they move onto Debian Stretch, they can get H.265 GPU accelerated video decoding working. It's been over a year since Stretch was released, which is an acceptable amount of time for those distros to update, and they have... except that those older distros (Ubuntu 14.04/16.04 LTS and Linux Mint 17/18) are still supported by Canonical, Mint, etc. However I'm also going to go out on a limb here and say it's likely the amount of users running those older distros affected by the proposed change might be minimal. Having to wait for those older distros' support to end would just slow MC's development progress. They also couldn't update to Stretch or Buster next year once that's out since the support for those older Ubuntu/Mint releases is several years. IMO, not a good idea to let those dictate (or slow) the progress of MC... gotta draw the line somewhere.
Still it does make sense to jump to Stretch for the amd64 build since enough time has passed. Keeping the i386 build on Jessie for the time being for the sake of compatibility for those distros makes sense. Maybe once MC25 development starts, start requiring Stretch for the i386 and arm builds as well. Having the amd64 build of MC24 target Debian Stretch along with Ubuntu 18.04 LTS and Linux Mint 19 (aka the latest LTS releases) unofficially would work - I can update the tutorial informing users of older Ubuntu/Mint distros about that change along with the recommended way to switch to the i386 build.
Having the amd64 build based on Stretch showing a message about it not being able to be ran on older distros might be a good idea, so users know to install the i386 build instead.