The "old-man" sentiment is simply outdated and needs to be put to bed with this latest version.
The SL63 is a capable performance car that puts out numbers that are close to identical with its GT brother. If the SL is not that great of a product, than neither is the GT. Both share the same under-pinnings as you indicated.
The number on the sales chart look good for the GT but the GT comes in a 4-door Coupe variant. The chart is probably lumping the two together, therefore inflating the numbers. If the chart broke the numbers down it would look quite different for the 2-door GT. The 4-Door GT is selling well.
I have seen numerous charts indicating that sales of convertibles, (no matter the price) have dropped across the board in the US since 2004-05.
There might be a few exceptions but droptop sales are not that great to begin right now, so with that taken into consideration, the SL is probably doing better than expected. These are older charts and articles but I believe still apply today.