You can understand why Sinclair doesn't want to sign a new contract, especially with how well Origi is doing, he would be well down the pecking order and now has Ojo coming through.
We have had a bad run of injuries with our strikers this season, so need some cover
To be fair, I think our strikers injury situation was a very extreme case this season. We had Sturridge, Benteke, Origi, Ings and also Firmino who can all play as strikers. But we were without Sturridge, Origi, Ings will through a period where we had a large amount of games. As a result, we were relying on Benteke (who has struggled all season). We did figure out that using Firmino as a no.9 was a good option to have and it worked well for us. Infact, it actually helped get our season, in particular in cup competitions, going again.
My point is though, if any team loses 3 first team strikers, they're going to be extremely light in that position. If Man City lost 3 strikers to injury, they'd have no one left. You can definitely have too many players for one position. Especially if they all want to play regular games. That's where you can start to see negativity creep in.
If Origi is going to progress beyond talented young striker and become a more regular part of our side going forward, then you've got to fills the gaps that have been left.