1. The main questions are still "Will this change allow for faster processing speeds for data requests?" and "Will this change reduce immediate resource demands?" One may be able to offset the other, but if both are true, then why not?
2. Support for MariaDB certainly seems to be growing recently, and as MyISAM is no longer the default driver for MySQL then it will become increasingly problematic as a useful tool in any app that uses older versions MySQL. Users will have to update their DB at some point and as we have seen with the demands on updating PHP in the transition from v1.9 to v2.0 this is not always easily accomplished- usually by Users on shared servers.
3. While hdd space is marvelously cheap, and is still declining, devs should still be mindful of it. My understanding is that deletes in MyISAM are left as blank spaces until the DB is optimized, which takes time and processing power. As Moodles do not get smaller over time, this can become somewhat problematic if an overnight cron is not properly set up- which leads to yet another problem with MyISAM.
4. MyISAM may be OK for organizations that have large numbers of reads and smaller numbers of writes, like schools and the like, but increasingly, the peaks and valleys of demand that traditionally flow with the day/night cycle are slowly being eroded by the nature of the changing learning world. I am certain that international enrollments, say at something like the OU for example, are increasing, so the time window when the optimize code can be run to refresh the db is, without significant impact on User experience, becoming smaller. This may not be an issue today, but I strongly suggest it will in the not too distant future. Internationalized MOOCs may or may not be becoming more popular, but Moodle should be able to accommodate them so can it do so while retaining support for MyISAM?
Please Note: I don't know if any of this is relevant to Moodle, not being a dev it is hard for me to say, but these are the sorts of issues that I would be considering I suspect, if for no other reason that to ensure that my app is remaining constant, or consistent, with identifiable technical, social, financial developments that will impact upon my app.