This is going to sound silly but you paid 800k in profits, but now have 4 years of banked costs you can use to _reduce_ your profit margin.
So you pay taxes on 800k profits, but then each subsequent year you reduce you profit by 200k, even though you don't have 200k leaving the door.
If 1M eng salaries was your stable state, then after several years you're... going to have 1M in costs to subtract from your profit! The stable state is the same!
I'm not going to argue about the capex change being "good", I do think it's worth highlighting that for large enough companies you're now looking at a different flavor of tax flow. Amortizing your costs over 5-10 years is something people like doing for other costs after all.
A few large companies with big cash stockpiles and profit can eat this the first couple years, not so for startups and companies with thin margins.
This is why it's bad. Large incumbents can manage this and then in steady state it's the same.
But for any new entrants that need to rapidly grow their engineering teams, it's a huge disadvantage.
We don't need more things in the tax code that protect large incumbents at the expense of new entrants.
It becomes a cashflow issue for startups. While the stable state is the same (not really the same, because of how companies evolve etc), cashflow issues in early days means $$$ from the VC money that I could've used to grow the company, now goes towards taxes for 5 years. That could be make or break for small companies.
If you have a pile of cash that you have no apparent use for, or can live without, yes, it makes no difference.