Is this reasonable to expect?

No... its not. There are two people in each lane. One of them has to lose their tower first.

To be honest, yes, this is expected by a mass majority of ranked players and is indeed reasonable to expect. I expect this as well, I expect my team mates not to feed and if they're playing a solo lane (top, mid) they can carry their lane early game along with the help from the jungler.

I agree with this post however:

A. I can excuse missing one or two MIAs simply because I ward lane constantly (I buy at least 2 wards and if I can I'll get 3 and keep them stocked until I can't anymore) so I should be looking at the map and knowing who is going to come gank me (assuming they don't have teleport and tp somewhere I can't see them).

B. While I hate losing tower I don't consider it the end of the world because if I let their wave push I can freeze at tower and farm a large wave with little harassment while being pretty close to base if a threat presents itself. Losing a tower does tend to bother me more if I'm mid, though. However, being top or bot lane I don't sweat it too much as long as mid either has taken the tower or has a different advantage (gold, good wave clear, good zoning…)

I definitely agree with wanting to not feed the enemy laner so they can roam especially if they have the power to instakill. I can forgive a couple deaths due to stupid mistakes because everyone makes them once in a while but there's a mantra I keep that is basically a blanket statement for everything listed here.

It's that you should always learn from your mistakes and if you aren't doing that then you should start or I'm just going to ignore you so I don't have to play with someone who can't or won't learn from their errors. If you die and you understand why then you should die that way again.

btw we haven't played a game together, yet, I don't think. We should do that some time.

I actually think that this is not reasonable to expect. I liken LoL to bowling. Normal games are like just screwing around with friends and ranked games are like bowling in a league. Do people take bowling leagues more seriously? Sure. But you know who they allow into a bowling league? Anybody. Terrible bowlers are allowed to join leagues too. Likewise, terrible LoL players are allowed to play ranked too.

I appreciate that it's frustrating when you get paired with a terrible player. If, however, everybody has a right to "expect" teammates who are at least average, then all of a sudden, those below average players are pariahs. Frankly, that sounds like the situation as it stands right now. Everybody believes that they have a right to teammates who are at least average and rages when they're paired with a below-average player. If that's the alternative, then no, I can't endorse that expectation.