It's my guess that the chances of the string tearing are next to nothing. In all my years of wearing tampon, I've never had that happen, and I've probably been using them for at least 30 years. I did, however, forget to take the last one out one time after I had my period. I even had sex several times after that yet with it in, and I didn't even know it. What gave me a clue to a problem was the odor that was coming from my vagina (not pleasant!). Right away I thought that I might have left a tampon up in there, so I had to go fishing around for it. If something like that does happen, just don't get all worked up about it; you'll be able to get it out. Just reach up in with your thumb and forefinger (use lube if necessary), take deep breaths to relax (it's really no big deal) and feel around for it. It took me a little while to get mine out, but then mine had gotten pushed up in their pretty far with my husband's penis.
I did get it out though by myself, and I'm sure you'd be able to too if you'd run into that situation of having to go poking around up in there. Even on the slight chance that you wouldn't be able to get it out, there's always the doctor. I'm sure they've seen and heard it all. I told mine about an embarrassing problem last week when I was in to see him, and he said that that was nothing, that he doesn't think there's anything he hasn't heard his patients tell him over the years. So just when you think that you'd be going in for something that he's never seen, trust me, he has.