So ...something decreases in size by, say, 50% every, say, 12 months or whatever.

It will, therefore, never cease to exist. It's eternal.

Right?

Of course, all this super simple stuff doesn't take long to appear devilishly complex & fascinating to the likes of me.

Zeno's Paradoxes:

http://tinyurl.com/8xtcy

Thomson's lamp:

http://tinyurl.com/lhrlmz