It has been four years since Roger last won a slam. However, just because of that we cannot write him off. The important point is that he is still remaining a contender as evidenced by his finals appearance at 2014 W, 2015 W, 2015 USO which are not in the very distant past. Even this year, he reached SF of all the slams he played. That is not shabby.
I peered through history books and found out there is still hope for Roger. For example, Boris Becker won his penultimate slam at 1991 AO and then he won his last slam at 1996AO. During those five years he was finalist two times (both at Wimbledon, lost to Stich in 91 and Pete in 95). One small caveat is that Becker was just 29 when he won the last slam at 96 AO. But, I am focusing here on the gap between penultimate and last slam of players. The second highest such gap (among open era players with at least six slams) is that of Pete who had 2year and 3 months gap. He did make the finals twice at USO during the gap.
History also has several players who kept playing for a lot of years after their Slam. However, typically they were not relevant and did not quite remain as contenders (unlike Roger). Jimmy Connors played for 9 years after his last Slam, but reached only one final in those nine years. JMac played for 8 years after his last slam and reached only one final in those eight years. Wilander player for 8 years after last slam and he did not even reach a final during those 8 years. Lendl played for 4 years after his last slam and in those 4 years he reached only one final. So, as you can see Roger is doing better than all these people who played for several years in vein after last slam. So, the players who hung on in vein had reached only one or zero finals after their last slam. But, Roger has reached three finals.
So, I would not write off Roger. Having said that I would not bet on Roger winning another slam.
In other words, it is possible although not quite probable.
Also, Boris Becker example shows it could happen at the last place you expect as well. Becker's result at AO the previous four years were terrible (3rd, 1st, did not play, 1st). The conventional wisdom was that he will win at Wimbledon again if at all he does as his performance there for five years after his penultimate slam were great (F, QF, SF, SF, F), but he never won it again.