Why must the goal be everylasting? (Anicca) Why must someone strive for something which is everlasting, e.g. Nirvana? It is unfortunate if everything is impermanent but I think this is a mistake, a false conclusion, that you shouldn't be attached to anything which is impermanent. 
It is not 100% awful if everything is fleeting. One doesnt need to cling to a belief that there is something which lasts forever. Did anyone come to the idea that everyone cling to the buddha and the idea of liberation? 

Well now one could argue that you can enjoy something  which is impermanent and not get attached to it, but that is never really mentioned in the suttas.. it's not about enjoying but always to renounce. Now, I thing that it's not  good to base one's goal PRIMARLY to self oriented ones, i.e. only achieving enlightenment. I know that this is not a good motivation and that one rather focus on others or on much  measureable goals like.. meditating twice per day or showing forgiveness or to apologize for one's wrongdoings.. Agreed on that? I think you all do but still the main goal in Buddhism is to escape Samsara and the cycle(s) of rebirth. Which leads to my second question:

Does one practise metta just for the sake to cleanse one's karma, for the wish that the other escapes samsara (which is ok if one literally - and not metaphorically - believes in it) and to not being reborn in this - im sorry - "mess" yet again? 

I think, it is better to stick to an agnostic "i don't know" attitude concerning rebirth, samsara, nirvana or rather reinterprates them than to greedily chase for liberation which is just another craving and could foster laziness, selfishness and everything which is contrary to the teachings.
It is unfortunate if everything is impermanent but I think this is a mistake, a false conclusion, that you shouldn't be attached to anything which is impermanent. It is not 100% awful if everything is fleeting.

That sankharas are impermanent seems to be an observable fact as well as theoretically plausible.

Whether that seems "unfortunate" is more debatable. (reference)

Also FWIW I think that the Buddha defined "unfortunate" (in the first and second noble truths) without mentioning "impermanence".

Does one practise metta just for the sake to cleanse one's karma, for the wish that the other escapes samsara (which is ok if one literally - and not metaphorically - believes in it) and to not being reborn in this - im sorry - "mess" yet again?

I think it's better for everyone, isn't it -- better for you, for me, for everyone else -- if for example I have metta towards you than if I were to hold ill-will. And better now, not just in the future.

There's the eightfold way, which starts with right view (i.e. wisdom). And there's the three-fold training, which starts with virtue and ends with wisdom. It's a bit like a chain, you can see how it works by examining it from-front-to-back or from-back-to-front ... it seems to me that metta is associated with virtue and wisdom, i.e. it's "right" and "wise".

But I think it's practical. There are many articles written about the brahmaviharas, for example:


  These four attitudes are said to be excellent or sublime because they are the right or ideal way of conduct towards living beings (sattesu samma patipatti). They provide, in fact, the answer to all situations arising from social contact.


That article says quite a lot while hardly ever mentioning karma and samsara.

I think, it is better to stick to an agnostic "i don't know" attitude concerning rebirth, samsara, nirvana or rather reinterprates them than to greedily chase for liberation which is just another craving and could foster laziness, selfishness and everything which is contrary to the teachings.

I mentioned it in a previous answer yesterday, but in case you didn't catch it there, I think the idea of "greedily chase for liberation which is just another craving" might be addressed in this sutta: Brahmana Sutta (SN 51.15).