The thing I love about BattleTech that I found quickly is that it many ways it is a game about optimization.
The Battlescore, for example, is only how great your mech is in most situations. It gives a very vague idea of power but isn't everything. It is entirely capable of winning a fight with a mech with a much smaller score.
So I was looking through mechs and I found one with a dismal battlescore and kind of dreadful stats, as it could barely move... had a walk of about 2 and it was a very light mech. I asked my Battletech friend "What is the point of this mech?" and he told me it takes advantage of city cover combat. In fact it makes this Kickstarter weird in a lot of ways because the mechs it includes are mostly for use of the player when there are a lot of great defensive mechs that would be used.
It is really telling when the people who made this kickstarter bad mouth vehicles and basically say "if the stars align vehicles can win", when those with experience know that vehicles can be outright scary (sure they are fragile... but when you get marked and suddenly take a metric ton of homing LRMs up your butt by a vehicle that can travel freely around the map...)
Looking back at the OP, the fact that they're setting it in 3025 (No frakkin' Clans!) is a definite plus for me. But that means there's even less canon justification for modular weapons. Yes, custom loadouts are potentially a thing, but in-universe you'd need to find a skilled Mech engineer to tweak a standard Mech, and then there's a pretty good chance they'd just fuck it up and you'd end up with a Mech with "quirks". Or find one of the rare House variants, which typically changed out a couple of weapons and maybe a slight difference in the armor levels.
Actually still happened and was extremely common because of travel times, the overall prices, and dealing with random chance. House variants occurred when a house itself had a variant and mass produced those but otherwise custom mechs aren't not only not unusual but expected.
As well switching load outs in order to deal with certain conditions happened quite a bit as well.
But THIS is just for individual companies, gladiators, and mercenaries. If you were in the military (and not one strapped for cash) generally speaking you went for the mass production market.
At least that is my impression from actually playing the RPG. It was Risky but honestly, even will a less experienced crew, losing a weapon was a LOT less costly then losing entire body parts or mechs.