Williams dragging their heals over what they want to do I guess. Probably want someone with backing who Massa kinda aint at this point.
Massa and Williams have both said they won't be racing for Williams next season. There's no 'likely' about it.
Last I heard/read was that Kubica is bringing about £7-8m in sponsorship but Sirotkin is offering double that. Except Sirotkin's backers have links to one of Putin's Cabinet members who's under suspicion of fraud and embezzlement so they're naturally wary of taking the money should something happen and the accounts of this chap get frozen or they get dragged into any of the fire. I
think they want to take Kubica, and have asked if he can get any more sponsorship, but with Williams performance last season and the season before not exactly setting the world on fire they NEED to make a positive step forward to ensure they're not hugely out of pocket. Payments for season performance are delayed by a year to ensure no one turns up and invests then leaves with their money. In 2015 Williams finished third, which as can be seen in this image:
meant a decent payout from the FIA at the beginning of 2017 along with their historical payment. In 2016 however, Williams finished 5th, which could mean a drop of £10-15m or so, maybe more from the FIA when they make their payment to Williams early next year. You can see why making that back in taking Sirotkin is tempting and he was, once fuel load, time of run, track conditions and tyres were taken into account, faster than Kubica apparently.
If Williams want to make any improvement in their car, and no doubt Paddy Lowe has brought some knowledge with him from Mercedes, they need cash. IMO if Kubica can bring more cash to match or get close to Sirotkin's offer, they'll take him, if not, Sirotkin will get the drive.
There has been some talk of an offer from Rosberg and Kubica for a 'trial period' of maybe 6 races for him to prove himself capable and if they don't meet certain criteria Sirotkin will take over. Not sure if that's really going to happen mind you, but time will tell.
Also regarding the payment chart above, I think it's truly scandalous that the payments to teams are so out of whack. IMO every team should get the same, the big teams will still make more through sponsorship and 'performance points' at the end of the season. It also shows how amazing Force India and Toro Rosso have done with their respective budgets, FI especially.