Several factors. Past performance being just one. Age, fitness, attitude are others - there may be some I've missed.
Past performance includes his recent past as well as his peak years, and gives some indication of a trend in his performance levels. Age and fitness are obviously going to influence him physically, attitude is one of the reasons I was concerned about his recent behaviour. All contribute to his effectiveness.
There's no doubt in my mind he's past his peak physically, he used to be able to beat three or four now it's rare to see him get past one. I saw a distinct evolution to a more team-orientated player earlier this season than the largely self-centred one of his peak years, but really he's been off his game in every respect since his injury.
Perhaps he'll regain some fitness and become more of an asset again, or perhaps he won't - who can tell?
When he was banging in 30 a season he was irreplaceable, if it drops to below 20 then not so much. Though still not too shabby, there will be others who can match it. After a while the law of diminishing returns starts to kick in, given the massive wage (for LFC) that he's on.
As I've said several times, I would sell him only if we have an adequate replacement.