Why shouldn't they? He has done quite a good job this year. He is not the MS of old but he's far better than a lot of drivers on the circuit. Take into account as well that Rosberg is quite an underrated driver. I don't think he's far off MS's level in his hay day, though I don't believe he is as good as him.
Things that have made Schumacher look bad this year especially is the failure of his DRS in quali 2 China so he wasn't able to put in the laptime to make it to Q3. He has been quite unlucky. Overall he has done a quite outstanding job and was quicker than Rosberg in Monaco during the race as well, at one stage he was the fourth quickest car on the circuit apart from the leaders.
This is from the FIA Lap charts for Monaco and I took it from a user on another forum I use.
Lap Chart
Race history
Lap Schu Rosberg
13 01:23.4
14 01:21.0
15 01:21.0 PIT
16 01:21.9 01:26.3
17 01:20.9 01:21.4
18 01:20.2 01:22.2
19 01:20.3 01:22.5
20 01:20.9 01:22.5
21 01:23.1 01:23.8
22 01:23.9 01:23.6
23 01:21.1 01:20.9
24 01:21.4 01:21.6
25 01:21.9 01:22.0
26 01:23.0 01:23.1
27 01:22.5 01:22.3
28 01:22.7 01:22.7
29 01:21.0 01:23.2
30 01:19.8 01:23.1
31 01:20.0 01:23.7
32 01:21.6 01:24.0
You can see that Schumacher is comfortably faster than his teammate after the first pitstop. He was lapping 2 seconds a lap quicker whenever he had some air. He overtook Rosberg on Lap 28, after which he really pulled away from Ros and built up an 8 second advantage in about 3.5 laps. Now you might argue that Rosberg was on the softs while MS was on the super softs, so here is another comparison :
Lap Rosberg Lap Schumacher
47 01:20.6 17 01:20.9
48 01:20.4 18 01:20.2
49 01:20.5 19 01:20.3
50 01:20.4 20 01:20.9
51 01:20.7
52 01:20.8
I'm comparing different times in the race here, but with both drivers on SS tyres and with a little bit of space in front of them. Rosberg, inspite of having a full 30 laps lesser fuel than Schumacher was lapping slower than Schumacher's first SS stint on a near full fuel load. Proof enough that MS was pretty much blitzing his teammate at Monaco and would have ended up a few positions up the road if not for his car failing on him.
Not bad for a 42 year old at all. Schumacher should keep going while he is still able to put in performances like this.