Most of the arguments from startup people about these rules come down to "this change costs me a lot of money" rather than "this change is bad accounting." Does anyone know what the arguments are to support the latter?

When this rule was changed, it was framed as eliminating a tax loophole: R&D work is basically a capital investment, since you are effectively buying and improving intellectual property during the R&D process. That suggests that this sort of expense really is a capex that should be depreciated over the life of the intellectual property rather than an opex. I personally think that this is a compelling line of reasoning.

I think there's a good argument that a forced 5-year amortization schedule is far too long for something like a random SaaS, but I'm not sure if I have a good argument that this is bad accounting otherwise. I don't expect that the IRS will be all that sympathetic to Silicon Valley complaining that one of their favorite loopholes is gone otherwise.