Yes, a bombardment function when blockaded (no space defense for a while, no shipyards, etc) is indeed needed, at very HIGH tech levels.  I've just been playing around with a previous game's save file where I had a tech victory (so I can fargin better understand the techs, the tech tree needs more tool tips, to give more info).  So, I was experimenting with plantary defenses, now that I understand that mechanic (thank you GC3 Wiki Writers!)....with the appropriate techs and 2 buildings (a prep center and planetary defense dome), you can have +100% planetary defense, which completely protects you from invasion.  Thats not a ridiculous cost.  Especially as the attacker has been B-lining down the invasion tech tree to make invasions "less costly", you have now thwarted their efforts completely.

Either a bombardment function, or a tech to "counter" planetary defense to a small extent.  There doesn't seem to be a tech to do that.  Soldiering mainly counters resistance and low levels of planetary defense.  In fact, there is a bug with Planetary Defense Dome, which is an upgrade of Planetary defenses.  Once upgraded, it is "now" a +3 Military Improvement, rather than the original +1.  AND, once a Defense Dome is built, you can actually build another Planetary Defense system.  This is surely a bug.  So, with 3 buildings, Planetary Defense Dome, Prep Center, and the "other" Planetary Defense, thats 140% Planetary Resistance.  And finally, just add an Elerium Defense Shield, and with these 4 buildings adjacent, you can get a 240%.  And if you want to go higher, put them on Military Terrain tiles or add other military buildings around them.  Insane, when there is nothing to counter once you get to 100% anyways. 

And on an Ideology Spam note - with the Terran Race and Pragmatic Builder II Coordinator ideology, you can continue to have ALL of your military production available at the far rear of the front lines, due to the 0% decay. That will counter well, the idea of blowing up shipyards and sitting on planets.  And just before you take my last shipyard (if you get that far), I would swap planets with another race, and do the same tactic to you on your unguarded flank Of course, I wouldnt have the pop to invade

Well, as others have pointed out, there are plenty of exploits in the Malevolent Tree.  I suppose, score one for the Pragmatics.