Thanks all for the inputs.

Just my 2c: since, as I understand, manufacturers of Android mobiles are making money only from hardware, that is, the handset, they have little inclination to keep releasing OS upgrades. This would simply allow users to hold on to their handset much longer. So it seems with any manufacturer (SE, Samsung, HTC etc.) the lifespan of a handset is 12-18 months, the period in which it's getting OS updates.