That assumes that the purchaser has more than one general use for the mech in the first place. There's not too much call for a forestry mech at a dropship port, or a heavy duty construction mech on a family farm (something that can be rigged for light construction work to assist in raising a barn, sure, but not something you'd use for building a skyscraper).
In which case you'll see the farmer renting out his HarvestMech at the DropShip port or with a construction company when he's not using it to harvest. A nice side source of income.
(Or, more likely, vice versa, where the farmer rents buys the harvest configuration pods and rents the Mech at harvest time. He pays a much lower price than if he'd owned it, and doesn't have to pay maintenance during the 10-11 months per year when it's sitting in the barn collecting dust.)
And for those agencies that use multiple types of industrialmechs, when it comes to buying 20 standard mechs vs 15 omnis, maybe they want 20 mechs for a reason, even if they're going 10 and 10 of two different types because they can run that many mechs simultaneously.
Really, it's about buying 10 Omnis or 30+ specialized mechs. For construction, for example, I can use a demolitions config, then switch to a site prep config as I dig out the foundation, then go to a cement mixer (or plascrete or whatever) and pour the foundation and then finally switch to a version with lift hoists, spot welders, and rivet guns for construction of the new building. I can even walk my mech out in the morning with cargo containers full of materials, then reconfigure on-site to assemble those materials.
Every configuration is another chassis, fusion drive, cockpit, mechwarrior and tech team that you don't have to pay for separately. And perhaps you're right that sometimes you'll need every specialty in the same numbers and at the same time. And, let's add, with no fluctuation in demand. Most of the time, especially on a small colony, none of that be the case, but it's certainly possible.
Even in that case, when the lift hoist is down for repairs, then so is your IndustrialMech. Meanwhile, when my Omni has the same problem, I swap out a spare pod and am back up and running in minutes. I also get the benefits of standardization in my spare parts stores, not modeled in StratOps but a realistic problem.
At let's say 5 million per mech, I pay 62.5 million for 10 omnis (plus config equipment), rather than 200 million on 40 specialized mechs. Then, I'll save on the 30 extra mechwarriors I don't have to hire, 30 techs (and 180 asTechs) that I don't have to pay, thirty times the monthly spare parts cost that I don't have to pay, savings I'll see year in and year out. And I take fewer revenue hits due to machine downtime. Any of that is well worth 12.5 million c-bills.