Major players are far more likely to get bought out and converted rather than fold (see: RocketMail --> Yahoo, HotMail --> Microsoft). Also, nobody's losing their warranties on their GMs. Similarly, if someone bought Yahoo, Yahoo Mail users wouldn't be left out in the cold, they would either be maintained or converted. Because users are some of Yahoo's key assets.
Nobody's saying there's something inherently wrong with smaller providers (viva la choice!), just that they come with inherent risks. I've lost accounts at places several times, back in the day. Of course, Yahoo or Gmail or Microsoft could delete your account tomorrow (either by accident or claiming you violated terms) and you'd have little recourse. So online mail always has some risk, which is why I archive it every so often. So I guess if you do that, doesn't necessarily matter.
Still, what's wrong with Yahoo or Gmail?
Keeper of the Magic Nickel