Why? He probably isn't going anywhere. If we come knocking with £10m in a 5 or 6 weeks I'm sure we'll have no problem getting him if we really need to.
If we want a Spanish striker closing in on age 30 Roberto Soldado would be an infinitely better signing and probably wouldn't cost a great deal more.
I wouldn't be against signing Michu as a last resort - I just think there are still many options to get through before he becomes our first choice.
Soldado's more mobile than Michu. Apart from that, they're even. Plus, Michu's 27, Soldado is 28. Finally, Michu's game is about technical skills, intelligence and strength, both in the air and on the ground: those traits age slower than, say, pace or agility.
I'm willing to wager that there's five, maybe six years left at the top level for Michu. I wouldn't say the same for Soldado.
No doubt, Soldado is a great striker. And as I said before, I'd be happy to have him at the club. But Michu has more value for money (if you're thinking long term), is already used to the Premier League, used to playing in a fast, direct passing side at Swansea, and is powerful, good in the air and good at hold-up play, all of which suit our current playing style (although someone slightly more fluid would suit our front three more, imo). Plus, I'd wager Michu would be cheaper than Soldado: not 10 million like you describe, but certainly in the 13-16 million range, which compares favourably to Soldado's 20 million price tag.
In my eyes, Michu would be almost neck and neck with Soldado in terms of the quality we could bring in: certainly not a 'last resort', as you term it.