Diversity (used in this context) is not a 4-letter word, and you're correct that 'single-sourcing' is generally not a good position to be in in most any context / situation.
^ This. The "all eggs in one basket" risk. Applies equally well to power sources for lighting.
That said, I do happen to have only a couple of lights beyond my 18650-driven Malkoff units. A 4Sevens Quark that runs on CR2, and a D-cell prototype LED from LambdaLights (a 2DXMLPU MagLite 2D). Don't use either of those much, anymore. The everyday units I use are the Malkoff drop-ins. Very good reflectors, high reliability and quality, sufficient lumens for my purposes.
Still, the risk of non-availability has meant I have instead boosted my personal inventory of batteries and upped my frequency of recharging, so that it'd be a long time before I'd need an alternative, even in the event of zero power and zero 18650 battery availability. So, in practical terms, unless we have another Carrington Event-level of solar storm impact (or similar alternate problem creating widespread impact upon power generation), it's unlikely that I'll be faced with lack of lighting for quite some time to come. Even with just the 18650s as my power source.
Even if push were to come to shove, where the SHTF were so bad that all human-crafted lighting became non-existent, we'd be able to fall back to basics: rising and sleeping with the changes of sunlight. If it got that bad, I figure we'd have much more to worry about than just a bit of nighttime portable lighting. Humans survived prior to such technologies. History's proven we're a fairly adaptable sort. We'd get past it. Adapt and overcome, 'n' all, even if without our precious preferred personal power sources.