I think it would be doable. Some people mention salary issues. Well, it's true Williams couldn't afford paying such a high salary to their driver in recent years, but hey, there are some ways to solve that issue. If we assume Santander would follow Alonso to Williams, it would be enough to maintain their current R&D budget and pay his sallary at the same time. Don't forget having such a marketable driver would also help to attract other sponsors, maybe getting an upgrade on some current sponsorship deals etc. Williams will probably also get more than ever money from FOM this year, with 4th or maybe even 3rd in WCC at the end of the year being very realistic prospect.
It's true this team is not proven to be capable of producing competitive cars consistently. But so wasn't Mercedes when Hamilton went there. So wasn't Red Bull when Vettel turned down BMW's offer in order to join them. If you don't risk, you rarely win. And Williams would be on a good way to continue an upward trend under the technical leadership of Symonds and Smedley, with additional sponsorship interest thanks to hiring a top and popular driver and getting their best reults from a long time, and finally, with a real benchmark behind the steering wheel, the most competitive and complete driver Williams had perhaps since early 1994, which could make a big difference when it comes to capitilising on a decent car and delivering results it deserves each time, which could prove to be a further boost to a team morale, sponsorship interest etc. A positive circle which could end up with maintaining their level closer to the sharp end of the grid for the first time in many years.
But well. What Alonso needs is to have a WDC capable car within a year or two. That would be a very long shot at Williams. They're fully capable of making a step forward, but perhaps not that big in that short period of time. If he feels comfortable enough at Ferrari, then I doubt it would be a good move for him. If he wants a new challenge, then Williams is a decent option for him. If he wants another WDC, well, perhaps staying at Ferrari and hoping they would be able to catch Mercedes when it comes to the engine performance would give him a better chance.
But you never know. Winning a WDC with Williams in these circumstances would certainly make him a legend.