It is wasting of money. You don't spend 30 mil on backup short term striker. If we want to spend on striker then lets go for real deal and go for Dembele, Werner or Belotti or some other high quality long term striker.
If we really want only short term option( because we believe in Greenwood) then lets just find some old proven striker. You have dozens of them who can be useful in these 6 months. Not to mention that Piatek went to bloody Hertha for 22 mil few days ago.
Also, we are not a team who have dream first 11 so that we need search for backups. I am huge fan of Martial but he is not Messi and Greenwood is not future Van Persie. Lets either believe in them or buy quality striker and let them fight for spot.
Value has changed. You do spend that much, when Bournemouth are in a relegation battle (and in the bottom 3) and there arent many other options for here and now.
Dembele, Werner and Belotti are not coming in January. You rarely ever get big signings like that in Jan and the need is for now.
Nothing stops us from going for these players in the summer by purchasing King. However not getting CL football more than likely ends any hopes of getting them or any top quality striker.
With regards to the dozens who can be useful in 6 months, sure. Name them though.
I have no issues with us getting somebody better than King if available, but who is it?
Who can do a short term job, adapt (or is adapted to the league) and will be a willing backup to United for 30m or less?
And as stated, if we want the two to fight for a spot, given they are both having similar records, its not the worst thing in the world.
With Spurs and Chelsea seemingly not bringing anybody in, that top4 spot is up for grabs. King would be on low wages, and would want to actually be at United. I cant think of many better players than him that would be willing to do the same.