Playing softly with no pressure means your lips are maintaining your embouchure without extra "help" from the mouthpiece to hold everything in place. An analogy is balance and dancing. What's harder, Yoga or salsa dancing? Both require core strength, flexibility, and balance. On the surface, dancing appears harder. It's faster, more "energetic", and requires moves that look more difficult -like playing loud and high. There's nothing fast, aggressive, or loud about a 30 minute Yoga session -like playing softly with minimal pressure. Try to do both for an hour, though, and see which one leaves you the most exhausted/sore. Yoga isolates your balance and flexibility, meaning that you have to be stronger to do it well. Soft playing does the same for your embouchure. Dancing provides a crutch that can hide deficiencies in both (e.g., movement through steps only requires balance for a few seconds, your partner's leverage can hold you up and support you when doing something on one foot, etc.). Using pressure and playing loud and high do the same for your chops. While dancing and playing loud and high can "hide" deficiencies in technique and strength, it ultimately backfires in just the way you labeled -endurance is crappy without solid basics.
To be the most effective at both salsa dancing and playing loud and high you need to be doing things that isolate and develop your core strength and balance. For dancing that's Yoga or some other form of balance/flexibility training. For playing your horn, it's soft playing working on flexibilities, scales, and tone.
Hope that helps.