This is the second day in a row I can use the same comment!
Products with >50% market-share are not irrelevant; they are VERY relevant. And as much as you might hate it, IE is not just going to die.
On the other hand, irrelevant products with disproportiate marketshare inevitably lose marketshare eventually. And the more marketshare you have, the less you can gain and the more you can lose.
Mediocre products with huge market penetration are more likely to die a horrible death because: 1) they have maxed out their growth potential, and there are fewer and fewer converts for them to target. 2) since their quality is bad, better competitors are likely to gain viral growth by only contrasting themselves to the mediocre status-quo; its name recognition will be used against it and its brand sabotaged (i.e. "Like X but doesn't suck", "a better X", "an X with ..", etc.)