'Arguably' is an understatement, to put it mildly. Tanks are awesome - they're just not for attacking cities. Just as is the case with the former cavalry. They're still kings of the battlefield though - nothing outside of stealth bombers, GDR or nukes can really stand up to modern armor. Your 'highly promoted infantry' will get steamrolled by an armor ditto if fighting in the open.
I agree that bombers are nice, though, especially if you're so far ahead technologically the opponent can't muster fighters nor AA guns.
Triple terrain promoted mech inf in front, getting flanking bonuses from more mech inf either side and behind would stand up pretty well when fortified. Then, use double attack and extra range promoted rocket artillery, to finish off the modern armor.
I try to take units through from warriors and catapults, to mech inf and rocket artillery and they get very highly promoted along the way. I suppose you could go from horse right through to modern armor, but mech infs would still have numerical advantage as the aluminium you need for modern armor is finite on every map and required for a lot of things.
I do agree that tanks are good in the open though, to clear out enemy units from around cities, before letting the mech inf come in and take the city down.
Personally, I'd rather be using my aluminium for stealth bombers than modern armor. Stealths have huge range, are great in attack and defensive roles and with a couple of promotions it doesn't take many to reduce a city's defenses to nothing. Then just walk your mech inf in for the final blow.
I still think oil is best used on bombers and aluminium is best used on stealth bombers.
Tanks and modern armor have their place, but if I have to use a resource, I'd choose bombers/stealths over tanks/modern armor any day and let my highly promoted, numerous and resourceless mech infs do the dirty work.