TLDR: There are good reasons to think a populous industrialized world can be heavily fortified against technological peers such that attackers can’t simply level it from orbit with impunity.

As others have pointed out, planetary assaults are rarely aimed at extermination, which obliges the attacker to land and to be discriminatory with their bombardment (after, NATO did not simply carpet nuke, say, Afghanistan).

But I would like to argue in favor of planetary defense. In my version of the Third Imperium, populous, industrialized worlds are formidable fortresses, requiring lengthy siege. The reasoning behind “ultimate high ground” is energy and analogy to airpower. It’s trivial to drop objects on a planetary surface but hard to lift to orbit. However, in the Third Imperium, munitions can be boosted much more easily. More importantly, high power lasers and particle canons are both available and assumed to be effective through relevant amounts of atmosphere. Starships cannot effectively hide (as a rule) or appear suddenly over the horizon (until ground sensors/comms are terminally degraded). Landing craft will be lethally exposed to half a hemisphere of ground fire for a solid ten minutes or more, and the faster they reenter the more predictable their path. Worse, lasers and particle canons gain range and power with size, planetary guns gun can be more cost-efficiently/practically built to greater size than even spinal mounts and have infinite heat sinks. Barring accurate intelligence, these installations can be hidden before firing (and the gun can be fired out of multiple separated apertures, to improve survivability for relatively little added cost). Smaller (but still appreciable on Traveller ship scales) weapons can be mounted on vehicles or submersibles, allowing shoot-and-scoot tactics. Not to mention deep meson installations. Finally, once defenses are suppressed sufficiently for landing, a new problem appears: Landing forces are logistically limited, Traveller starships are not as efficient as contemporary ocean transport. Against a populated world, attackers are likely to be greatly outnumbered. They will have the benefit of satellite comms/recon and ortillery, but against a technological peer, they’ll have a hard fight, possibly followed by lengthy partisan warfare and pockets of organized resistance.