Service S & G or ATU?

Hi people, I am now just under 600 km above the last service. So it’s about time …. My SLK (first registration 2000) has been maintained consistently at Benz. Since Benz has always promised me a guarantee (including grating), but this guarantee has never been redeemed, I wonder if it still makes sense for my old car to have the service continued at Benz. Since I am a member of ADAC, I don’t really need the Mobilo guarantee (or as it is called …). Ben z now wants just under half for the large range of services over 600 € and ATU. I have only had good experience with ATU so far. Therefore, I assume that they can manage the Sercive properly. I am now interested in what happens formally with the vehicle when it is no longer brought to service at Benz. The guarantee would lose, would also the value of the vehicle decrease? Many greetings and best thanks