When will it become advisable to upgrade to Android L, from a security and usability standpoint?

A

AC Question

I am vaguely aware of the Heartbleed vulnerability and some others prevalent in older Android models, but what I am really interested in is when, from your point of view, will it become seriously advisable to upgrade from Android version I/J/K to Android L+, from a security and usability standpoint, and altogether abandon older models?

I have an old ZTE device at home, which I don't actually use anymore. The device never got any upgrades beyond Gingerbread; one could of course always flash a custom firmware, though using anything beyond ICS is a pain on the device. Having used the Gingerbread device, though, I know quite a lot of apps aren't really available for Android version G systems. Are we going to see the same thing on dear Kitkat any time soon?
 

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