Blue's goal is to prevent such a situation — to fight on turn 127 and drive away the invader. If this battle is won, then it's possible to buy off the attack with growth, as he still won't reach the town. And if it's lost, then blue played worse and rightfully loses the game.

If the rule is introduced, then of course blue's goal will be to prevent the red-rush, but why should we make the game harder for blue and easier for red.

What matters is not the loss of the army, but the loss of the hero — if red's main hero ends up in the tavern, then he won't be able to red-rush blue's town purely based on movement points.

Red can do it, there are a bunch of ways how! I told you that Fireball happens before the first day of the week, then red will have full movement points, plus a red-rush can be done with scout heroes, and with a main hero with control even if he has half his movement points left.

Added after 15 minutes

The main argument against red-rush is not that red is weak, but that the impact of red-rush on the game outcome is noticeably less than the hemorrhaging it creates. Because the main red-rushes happen on scout heroes, which are killed with and without growth, or on empty castles. That's why ideas were often voiced to amend the rules along the lines of "if it doesn't affect the outcome of the battle". But due to the difficulty of determining whether it affects it or not, each time we decided to keep the simpler version "it's not allowed, period".

Reds are not weak, they are strong. If red were weak, people playing blue at color pick would choose blue color, and they wouldn't care about your so-called traditions.

A long time ago, there was a rule here that you could attack a scout hero if it doesn't affect the outcome; they only removed it this summer (and even then, I disagree with it)

In almost all tournaments, the host immediately determines before the color pick who will play red, and the other one starts the color pick. There was an experience when during a restart they swapped colors, and even that was changed during the tournament... because they understood it's not fair if one starts picking colors and can also decide to play red.

And regarding the inconvenience that red bought out an army and then gets denied by the ban on attacking towns, no one forces him to carry the army with him; go capture blue's town, make sure you don't have new growth on your hero, and blue won't have it either.