This is my take on the situation. Apologies in advance for being a bit negative.
Here is a graphical representation of Williams' performance since 1990.
The size of each bubble represents the championship position. The largest bubbles are the championship winning years (1992, 1993, 1994, 1996, 1997), followed by the 2nd places (1991, 1995, 2002, 2003), and so on. Each bubble is centred on the co-ordinates [year, Average Points Per Race].
For the sake of clarity, the Average Points Per Race was calculated using the present day points system. For one thing, it allows for comparison across all years and, because it is the most "granular" points scoring system to be used, should provide more meaningful characterisation of performance - being capable of differentiating between 1st and 10th is more meaningful than being limited to 1st and 6th only.
There is fairly clear stratification in the chart:
- If you average 20 points or more per race, under most circumstances you're a championship contender, and capable of winning races.
- If you average more than 10 points, but less than 20, you're a competitive car, but not championship winning material. Wins are possible, but only under unique circumstances (retirements of faster cars, etc...).
- Less than 10 points per race, you're making up the numbers.
The Williams chart is highly dominated by engine era. Pre-1998 is the Renault era, and its obviously the best. During this time Williams experienced a confluence of engine dominance, technological dominance (active suspension, etc...), attracted top drivers (Mansell, Prost, Senna), and sponsorship (Rothmans).
1998-1999: Renault leaves the sport, Williams struggle with rebadged engines (Mecachrome, Supertec). The quality of the drivers drops (Zanardi, Schumacher R.), diminished sponsorship.
2000-2005: BMW engine power propels them up the grid. A stronger driver partnership exists (Schumacher R., Montoya), and they land big sponsorship (HP amongst others). Unfortunately, the development goes in the wrong direction, e.g. the Walrus nose, and BMW leaves for Sauber.
2006-2013: Absolutely dire. Inexperienced drivers (Webber, Hulkenberg, Senna, Maldonado, Rosberg, Bottas) and some really bad drivers too (Pizzonia, Nakajima, Senna, Maldonado). No engine partner, and 4 changes in engine provider (Cosworth, Toyota, Cosworth, Renault).
2014-2016: Switch to Mercedes engine power. Massa joins Bottas in 2014.
Outside of the Renault era, and the absolute height of BMW power, 2014 was the best result Williams could have hoped to achieve. This result was a complete step-change, breaking a disastrous ebb that began in 2004. Williams benefited from the issues at Renault and Ferrari, and have clearly been the next-best Mercedes-powered car, until very recently.
Given Renault and Ferrari were under-prepared, and are making continuous and large improvements to their respective engines, Williams can't expect to finish higher than 4th unless some component of the team's performance is radically altered. Where would this performance enhancement come from?
They already have the acknowledged class-leading engine package.
Hiring a championship winning driver is beyond their financial capabilities, and not in their DNA. They hired Prost and Senna in quick succession, but have never hired a World Champion since. At best it would simply maximise the potential of the car, without redefining that potential.
Innovation doesn't appear to be their strongest suit either, as evidenced by the Walrus episode, inability to make blown diffusers work properly, and their current issues with tyre management and general lack of downforce.
Unfortunately, I don't think there is much scope for improvement, unless they hire clearly faster driver(s), develop a significant technical upgrade that no-one else on the grid is aware of, or take possession of an engine no-one else has access to.
What is particularly worrying is how badly they fare during times of "engine equivalence". If we do enter an era of relative engine parity, and couple that with static regulations, I foresee Williams being threatened by other teams as well.