In short, we already have a plan. DART proved that we can do it, and off-the-shelf rockets like the Falcon 9 have plenty of performance. All that remains is to wait until early 2028 when we get a proper fix on the asteroid, then we’ve got 7 or 8 months to prep and launch a mission before the window of opportunity closes.
You’re not wrong, but something like this is well within the capabilities of private companies like ArianeSpace or SpaceX. Also, the stakes are just a city-killer asteroid, failure is entirely an option when there’s plenty of time to evacuate the impact zone.