Again... you all keep handwaving "oh these modern Earth weapons are equal or superior to BattleTech", but you constantly keep ignoring the stated advancements in material sciences that BattleTech has. For instance, Endo Steel is explicitly stated to require zero gravity manufacture and "uniformly mix high-density steel with lower-density titanium and aluminum. Rhenium is also used in this process." And while yes, this is one of their "advanced" tech items, even standard BattleTech armor technology is considerably more advanced than what we have now, and protects against a wider array of weapons. BattleTech armor handles circumstances like REENTRY HEAT just fine, whereas we have to make specialized heatshields to handle it (again, remember Aerospace Fighters are designed to go from and to ORBIT and beyond under their own power and they don't have special heat shields to do it, just regular old BattleTech Armor!).
That's also something folks here and neglecting in their analysis. Earth has no technology or armor designed against BattleTech's weapon grade energy weapons. While Lasers are likely one more easily understood, PPCs are a complete outside context problem from a defense standpoint for Earth... and PPCs are both very long range, common, and POWERFUL. Even the Lasers are outside of anything Earth has, and likely do tremendous damage to most everything they encounter. A single Large Laser is an effective counter to pretty much all airborne attacks, since none of our aircraft are designed against that kind of firepower nor do they have countermeasures for it, and would utterly wreck helicopters.
And again, as per BattleTech lore, Earth doesn't have the range advantage that you all seem to think it has. Any weapon with a range of 8 hexes (ground scale) is stated to actually have "to the horizon" range or longer in the case of indirect fire weapons, and while they do lose damage and accuracy when shooting beyond their normal "max" range per the rules (which, let's be honest here a moment, makes more sense than weapons going from 100% effective to not existing that the normal weapons ranges imply) that still means that modern MBTs aren't just going to be shooting at Mechs without response. In fact, if you assume shoot to horizon ranges for things like lasers and the like, BattleMechs being so tall actually becomes ADVANTAGEOUS as it gives them a longer horizon and visual lines for firing their weapons.
And finally, as to massed artillery, that's why I keep bringing up aerospace fighters. They would be able to easily wreck any artillery set up and Earth HAS NO COUNTER to them. Everyone here seems fixated on BattleMechs and Dropships, but frankly, but those aren't the real threat: Aerospace fighters are. Earth has no weapon that can reliably knock them out of the sky, as again, while they might work on the same principles, there is nothing indicating that modern missiles, even these "BROACH" would reliably be able to work against BattleTech's armor and be able to knock them out of the sky.
Finally, this is ALSO working from assumptions of perfect knowledge. People here can go "oh this Earth tech counters/is more effective then BattleTech's X" because we have perfect knowledge of BattleTech's technology and can spend the time to pick out it flaws. In a fair scenario that's not the case: Earth forces would have no way to know that massed artillery is a potential counter to things, or that they would need to be trying to shoot down BattleTech's aerospace fighters with specially designed ANTI-TANK weapons. Rather they'd be forced to use what they have on hand, which would lead to serious issues... and even assuming they could get their hands on examples of BattleTech technology unless out in the deep periphery, they'd not really have the time to retool things to actually be able to counter the full might of a Clan or Great House.