I let the bike sit in the basement all winter without the use of a charger. If it starts in the spring, that's a good battery! If it doesn't, it gets a new one. Constant trickle charging can "mask" a battery with low capacity or internal resistance problems.
No, I don't think so. I think this is actually bad advice for everyone except the people that sell new batteries. Leaving a battery to slowly discharge all winter (they all do this) will significantly shorten the lifespan of said battery. Think of it this way: When a battery is "in use" it gets recharged every time you drive the vehicle. The batteries that last the longest are the ones in constant use, not the ones in someone's basement.
Keeping a battery well charged at all times is the best way known to extend it's lifespan. That said, I don't recommend just hooking it up and letting it trickle charge all winter long either. If you have a charger that shuts the charge current off completely at a predetermined voltage threshold then I suppose that's fine. Those devices are out there. But you don't want to be juicing the battery for 4 straight months otherwise.
What I have done in the past is hook up a low current charger for a day or two about once per week or so. Since I have a few bikes, 2 boat batteries (out of the boat in winter) a lawn tractor battery and one car that sits out winters, I just cycle the charger between each of those. If I forget to move it and it sits on one battery for a few days, no big deal. Just keep it moving and keep 'em all charged.
Now, back to the original posted question. The only way that you will get any early warning on an impending battery failure is if you have some real-time metering installed. Just a simple battery voltmeter will show you the voltage before you start, how low it sags after running the starter and how well it recharges after running. It also shows you that the regulator is running normally and not allowing the battery to discharge during running (the biggest cause of being stranded on the road) or that the regulator is overcharging the battery and cooking it. I heartily recommend some sort of voltage monitoring meter on any motorcycle. I have no idea why they don't just put them on all new bikes considering the demise of the kickstarter 20 years (and 500cc's) ago.
The catestrophic failures mentioned is a real possibility, but that is generally not directly related to age, and luckily, is relatively rare. It could happen to a nearly new battery. It's the slow sulfation of the plates that is the old-age phenomenon. This is the one you will be able to see occuring (in advance of a failure) on your voltmeter.
I would not (have not) replace(d) a battery from 3 year old (2005) prophylacticly unless I noted a lack in performance. Otherwise you're just throwing your money away. If well cared for, batteries can last for 8-10 years. If abused (allowed to deeply discharge, banged around, etc.) 2-3 sometimes.
Best batteries made? IMO, Yuasa FGM batteries. Yep, significantly more money. But you get your money's worth. Check their web site for all kinds of great (non-sales related) battery info.
Oh, one more thing: If you buy a battery in advance, don't activate it (put the acid in) until you need it. The shelf life of a non-activated battery is nearly indefinite. Once activated its sulfation failure clock is ticking...