How well does the Jumbo work when the RCT has to quickly retreat from the planet due to superior enemy forces? Especially when the assigned passengers and equipment are spread out across a continent guarding multiple locations (cities, military bases, supply depots, etc)?
The RCT unit commander would have to make the choice between abandoning troops, putting the DropShip and its passengers at ever increasing risk of being shot down each time it makes a pickup or assign a disproportionate number of fighters as escorts (thereby taking those fighters from other escort duties).
Not to mention the age-old mantra of 'Plan for the worst, hope for the best.' What if they discovered the defenders were stronger (or simply closer) than expected and they were possibly going into a hot LZ or needed to quickly deploy into combat lest the wave 1 forces get overwhelmed? Or discovered it after their LZ is suddenly in danger of being overrun? Using a single DropShip like you propose the RCT can lose a huge portion of its combat strength in an instant.
Well, I suggested 2 Jumbos and split the non-bay loaded Armored and infantry regiments to be split between them, putting all of 1 resource in a single dropship violates the dictum of a single point of failure. I also had at least 1 regiment, more likely a regiment and a battalion set up in vehicle bays to be able to be delivered straight into combat.
Couple of flaws with your scenarios . . .
First, 'quickly' is a misnomer in just about any troop movement. If you are being pushed off world, your AO is most likely a small area- not spread all over a continent or two, otherwise how are you getting pushed off world? If they are getting pushed off world its not something that is going to happen in minutes- likely tens of hours if not days. Which means vehicles- armor, troop carriers and support vehicles- can be loaded as the AO collapses. My list also included 4 armor combat transports if you used the Triumphs which IMO depended on collar space & consumables packed- so the Excalibur, Outpost and Triumphs could take the last of the deployed armor as the AO is abandoned.
Second, if you are getting pushed off a world you are not likely leaving with all the troops and equipment you landed with at the beginning. Since armor and infantry work well for defense which allows the mechs to be the mobile decisive element, you are likely going to have lost a chunk of armor in a failed invasion. Which means you are not going to need all that cargo space you used to ship in equipment . . . and you are going to leave with less ammo.
Third, if the LZ is getting overrun . . . the DS are going to load what they can and boost for orbit and what is on the ground might be able to perform a fighting retreat. If its bad enough the armor would be sacrificed in a rearguard action while the more mobile mechs and any cav armor executes the break out (think Syrtis Fusiliers on Tikonov in 4SW). If the troops in the LZ are getting overrun because they landed in a bad area (its what recon is for, kick those pathfinders out the side of a Small Craft for HALO drop) . . . you do not reinforce failure. Second wave DS with cargo shipped troops would not be landed, it is tactically and strategically unsound.
Finally, if troops land a dropzone in a contested area . . . you have already screwed up the operation. Look at Air Assault operations, for the most part helicopters put the troops in areas that are not actively defended at the outset. Reinforcements may come in at the hot zone but the point of such a movement is to safely put the troops on the ground within striking distance of a target without being vulnerable. For example look at the Blakist false flag op for Marantha in Double Blind- St Jamais dropship ops were properly executed. The threat of landing all over tied down defenders and prevented them from consolidating.