There's several other counterfactuals you're not considering besides "Ukraine keeps their nukes and doesn't get invaded". Like, what if Ukraine kept their nukes, but Putin invades anyway[0]? Either they launch to make good on the threat and break the nuclear taboo, or they don't launch, in which case they wasted a bunch of money maintaining a nuclear arsenal that isn't useful.

"Breaking the nuclear taboo" is a problem because the only advantage nukes have is deterrent. They are very good at killing civilians and terrorizing states into surrender[1]. But throwing a nuke at a line of incoming Russian tanks would be utterly stupid. They're just too damned big. And the more you normalize the use of nuclear weapons, the less that deterrent effect matters, even outside of the usual "mutually assured destruction" scenario of a superpower vs. superpower nuclear exchange.

The significant casualties you're ignoring are as follows:

- Wasted taxpayer money from maintaining very expensive missiles and nuclear material that don't actually stop invading forces

- Low, but not non-zero probability of a nuclear accident caused by mishandling the nuclear material in the weapon (e.g. that one time we almost nuked North Carolina[2])

- Extremely low, but still not non-zero, probability of escalation to superpower conflicts that would result in the destruction of major population centers in a matter of hours[3].

[0] Remember, Putin is dumb, he thought he'd crush Ukraine in a matter of hours. Do you really think nukes will stop him?

[1] e.g. how we got Japan to go from conditional to unconditional surrender by flattening two cities

[2] https://en.wikipedia.org/wiki/1961_Goldsboro_B-52_crash

[3] Yes, this is potentially survivable, if you happen to be in a concrete basement, aren't in the fireball radius, follow proper decontamination procedure, have uncontaminated food and water for several days, etc. You still don't want this.