Mitigating kinetic attack
Guys, the notion of using freighter/tug wedges to protect against kinetic attacks is predicated on knowing where an attack is likely to come from and getting the aforesaid wedges into the path of the incoming kinetic projectiles. If you don't know where they're coming from, you can't interdict. Under most circumstances, you are going to know the attack is coming; look at how hard the Alignment had to work to pull off Oyster Bay.
The wedges are designed primarily to protect infrastructure and the ships generating them are normally kept in close company with the major bits and pieces of infrastructure — like Hephaesteus or Vulcan — to be protected, not the entire network of communications and power satellites and other dispersed infrastructure. They are not envisioned as a means to protect planets from every conceivable kinetic attack; that is the function of the Eridani Edict. Obviously, if they can be generated on the approach vector of a kinetic attack, they will be effective, but the tugs which were supposed to be on standby when Oyster Bay came in were located to protect the space stations, and they failed in their mission because no one saw the attack coming in time to respond to it. The tug which managed to ameliorate the much greater degree of damage Sphinx would have taken without its efforts did exactly what tugs were supposed to do and, as has been pointed out, its single impeller wedge was too small to stop the wreckage from a single smashed space station from getting through at least in part. A space station, however, is a much smaller target, and the standby wedges are intended to provide point coverage for those smaller targets.
The truth is that any planet in the Honorverse can be killed in any number of ways. As Duckk has pointed out, only relatively sick puppies sit around thinking about inflicting such megadeaths, however. Measures are taken primarily to protect planets against less than extinction-level attacks precisely because trying to protect them against deliberate total destruction is going to be so difficult. Thus the effective defense against planetary genocide becomes reprisal, and hence the Eridani Edict. My point in the much earlier post being cited was that star systems protect their space infrastructure against deliberate attack and their planets again natural disaster in part by using interposed impeller wedges to kill kinetic threats. I never meant to imply that a planet could be completely protected against an undetected kinetic attack, and I believe I stated in several other places (as well as here) that an attack which is detected inbound can also be stopped by any number of cheaper alternatives.
I will also just say that I do not recall ever having said that "the destruction of planets was the norm" before the Eridani Edict. The reason the Eridani Edict was created was, in large part, as the horrified response to a highly unusual situation in which direct, genocidal planetary attack(s) had occurred. It was the very unusualness of the attack which provoked the Edict to prevent any recurrence; this is not something which the galaxy is suddenly going to find acceptable as a routine tactic just because there's no one left to enforce the Edict against the perpetrators, and anyone so frigging stupid as to try something like this is going to solidify opposition to his efforts pretty damned quickly. An awful lot of people seem appallingly willing to consider that Mesa is perfectly prepared to start knocking off planets 'if only' it can find a way to lay the blame for the attacks on the SEM, but I doubt very much that would be the case, if only because of the difficulty of doing that without it's coming home to bite the people who tried it and because if they did manage to create a situation in which people started planet-busting, the SEM would — with enormous regret but great rapidity — start busting the planets of anyone who tried to give them the same treatment in retaliation and the Grand Alliance is going to be able to do it better than anyone else for the foreseeable future.
We're talking balance of terror here, people. The Edict took the terror out of the "mutual assured destruction" format because there was a third party — the SL — prepared to enforce the specified penalty against either side, but the knowledge that the other side (or its allies) will do the same thing to you and/or your allies if you try it should be pretty darned deterring, too.