if mold breaker type abilities never existed, then yes! so having levitate isn't 100% fullproof from immunity to ground types. not to mention gravity.
and you have to expect that eventually there will be a pokemon with an ability that is more or less the same effect as gravity making levitate useless.
sure heavy metal is definitely inferior to those other abilities , but its definitely not useless. and i have seen people used heavy metal combined with heavy slam before and it DOES HURT! even more than a low IV gyro ball.
Max base power of Heavy Slam is 120.
Max base power of Gyro Ball is 150.
Both have variable damage, yes, but only one does not require you to sacrifice defensive potential to capitalize upon it. Heavy Metal may not be useless in the most literal sense in that it does do SOMETHING (as opposed to things like Slow Start, Defeatist, etc.), but its niche is so minuscule that it's not even worth considering. At the end of the day, Bronzong is a defensive/support Pokemon first and foremost; many offensive steel types outclass it entirely. As a defensive/support Pokemon, it would much rather have one of its two abilities that enhances those roles by making it easier to survive.
Also worth noting your example of Tyranitar only amounts to 40 base power, even with Heavy Metal. Heavy Metal puts Bronzong at 824 lbs, while Tyranitar is at 445 lbs. When the target's weight is over 50% of the user's, the base power is only 40.
Target's Weight - Base Power
50+% of user's weight - 40
33-50% of user's weight - 60
25-33% of user's weight - 80
20-25% of user's weight - 100
Less than 20% of user's weight - 120
And in that case...
252+ Atk Bronzong Heavy Slam (40 BP) vs. 0 HP / 0 Def Tyranitar: 102-122 (29.9 - 35.7%) -- 30.2% chance to 3HKO
This is assuming you're using full offensive investment as well, which is quite gimmicky, so in reality it'd look more like...
0 Atk Bronzong Heavy Slam (40 BP) vs. 0 HP / 0 Def Tyranitar: 74-90 (21.7 - 26.3%) -- 3.8% chance to 4HKO
Meanwhile Crunch from Tyranitar will 2HKO Bronzong.
252+ Atk Tyranitar Crunch vs. 252 HP / 4 Def Bronzong: 258-306 (76.3 - 90.5%) -- guaranteed 2HKO after Leftovers recovery
There's just really not enough reason to use Heavy Metal. You could argue it's not useless in the most literal sense, but it is effectively useless compared to better options.