Why?

Of course they will be able to live with it but certain people would be peeved.  I'd be annoyed if it happened to me.  It's not going to happen because I'm not on XBL but if I was. 

Sounds perfectly reasonable, five years is a long time.

Well, Sony can't because that would mean they would be able to change the PSN ID - which they can't.

5 years is a long time, but Microsoft should add something to secure your Gamertag forever. Like if you're really fond of your Gamertag and switched over to playstation, you could give Microsoft support a call and say you want this certain gamertag to be yours forever or something. A little additional work if you want to keep your Gamertag.
A lot of people counter arguements here is that "If you dont sign in once in five years, you obviously dont care." But like people have said, what if you changed switched consoles this generation and plan to switch back the next. My name for example, Liquid_Faction, unique name and I could remember where I first heard it. It was on discovery channel when I was like 11, but it was Liquefaction. I misheard it as Liquid Faction, thus my name was born. It has some sentimental value to me. But If I was an xbox user in the 7th gen, with the name "Liquid_Faction," and switched over to PS in the 8th gen, good chance with the same gamertag. I would rarely, if not at all, visit Xbox websites because theres no point. It would probably just slip my mind, and when 9th gen comes around and someone takes my gamertag, it would suck. But probably my case is just extreme and I'm just a little fond of my gamertag.

It takes five years and this gen is nearing an early grave. I am pretty sure it will still be there if you decide to switch back... 

Yeah I know, that's what I wrote. PSN was poorly set up, so you can't change PSN ID. But according to Vivster it would be relatively easy to fix so maybe they're just being lazy.

I don't care what Vivster says. We don't know the exact database scheme to say something like this and we don't know how the applications around that work.

Generally, this is not an unsolvable task, I know that, but without more knowledge this is the same like saying game x should run better because it looks bad, without any knowledge on how it works.

Lol this gen will easily be longer than 5 years.  

An actual active user matters more I suppose.

I'm sure many gamertags lost won't be taken as well.