A big point of splitting up is so that (in both directions, to the extent that it applies) cross-unit costs aren't baked into operations. Subsidies like you suggest directly undermine that.
Those aren't actually spinoffs in the sense of what IBM is doing; "Google" was effectively just renamed "Alphabet", with its core business in a new subunit called "Google". They are all still within the same corporate ownership structure. Its an internal organizational change, not a separation into separately-owned organizations.
A “spin-off” within a common corporate umbrella is a different thing done for different reasons than a corporate divorce kind of spin-off like IBM is doing.
[1] https://www.pwc.com/m1/en/blog/intangibles-tax-risks-opportu...
[2] https://prospect.org/economy/decisive-tax-defeat-for-the-mul...
Like: We'll give you an 80% discount in the first quarter, and the discount goes down by 20% per quarter, eventually you'll either be negotiating your contracts with us like any other potential customer, or you'll have moved off to some other platform.