Does the "PCs save the city from the kaiju" scenario actually work, given a lack of immunity to mundane weapons in the 2025 Monster Manual?

From what I can tell, Wizards of the Coast wants the city vs. kaiju scenario to be feasible. Page 51 of the 2024 Dungeon Master's Guide shows a CR 23 blob of annihilation attacking Eberron's Sharn (in a piece of artwork with a somewhat unique depiction of the city's skycoaches). Presumably, it is up to the PCs to valiantly step in and save the city from utter destruction. However, I am not so sure that this is viable, given a lack of immunity to mundane weapons.

The blob of annihilation is a CR 23 with AC 18, HP 448, and Resistance to Bludgeoning, Piercing, Slashing. It has limited AoE: just its Engulf with a 30-foot Speed. It does not seem especially unfeasible for a force of mundane mooks with mundane ranged weapons to brute-force their way past that Resistance and drop the blob. This is to say nothing of whatever magic-users the city's defenders have at their disposal, who can make (now non-spell) ranged attacks that deal non-physical damage.

The tarrasque, at CR 30, is a little better-off with AC 25, HP 697, Resistance to Bludgeoning, Piercing, and Slashing, near-immunity to Magic Missile (but not those pseudo-spell attacks that are not actually spell attacks), and better AoE. But even this is not impossible fell with mundane mooks, to say nothing of actual magic-users.

Looking more closely at the Sharn vs. blob of annihilation scenario, the City of Towers seems eminently well-equipped to tackle this sort of threat. The 5e books give Sharn a population of half a million, which Keith Baker personally multiplies by a factor of five or more. Khorvaire has just emerged from a continent-wide war, during which multiple CR 25 warforged colossi (each 200 to 300 feet tall) were fielded, so armed forces have experience confronting gigantic war machines.

I have a hard time seeing how Sharn fails to round up some mundane defenders and shoot the thing down.