That’s overstated, look at say elite marathon runners who do incredible amounts of daily exercise without bulking up.

A 500 calorie workout might only be 400 extra over spending that time sitting on that couch. Similarly extra muscle on it’s own takes energy but fat also burns energy so even modest weight loss balances that out.

Having lots of muscle mass isn't healthier than being slim. It's healthier than being fat, but being slim is going to be a lot easier on your heart, joints, and other organs.

(Have a look at people who are 80+ and fit. Most of them are slim. You don't see a lot of bulked up old people)

Your overall point might be correct, but your old people one is just wrong.

Old people aren't super muscular because you need high levels of testosterone to support musculature, and it naturally declines with age and particularly tanks when you are elderly.

The digestive system also gradually loses the ability to absorb protein with age. Elderly people actually need more protein just to maintain muscle mass, but most of them fail to adjust their diets accordingly.

Being overweight also raises your metabolism. Focus on the mass, not so much the rate.

Lean muscle at rest only burns about 8 kcal/day/kg more than fat. So the metabolic effect is pretty small.

At rest sure, but you gotta carry all that extra weight around when you do anything else.

Carrying around an extra kg of weight takes roughly the same energy regardless of tissue type.