Maybe something like:

$cache = ObjectCache::getLocalClusterInstance();
if ( $cache instanceof EmptyBagOStuff ) {
    $cache = ObjectCache::getInstance( CACHE_DB );
}

Changing getLocalClusterInstance() might be viable now as well, since most true cache-aside stuff has been moved to WAN cache.

There should perhaps be another config variable like $wgSessionCacheType etc. ($wgRateLimiterCacheType?), defaulting to CACHE_ANYTHING, that would be used in that function?

Worth mentioning: T147161, T160519 which resulted in Gerrit change #342959.

Really: should we support CACHE_NONE outside of development?

@brion @demon Also note that I have plans to deprecate CACHE_ANYTHING in favour of making CACHE_DB the default fallback during installation. See T115890 for details.

Update - I currently see two paths forward:

1. Always have a cache by default, adopting CACHE_DB as standard fallback, and making sure that stuff not worth caching under those conditions use something other than the main/wan cache interface. This is T186673.
2. Or, a possible interim/alternative solution might be to use MainStash as the fallback, e.g. instanceof EmptyBag ? $services->getMainObjectStash(), where MainStash is currently the only "has to be on" storage not affected by main=CACHE_NONE.