Q:

How can I counter the argument "there is better and there is worse"

Context
The context is "in general" but let's say that we are in any discussion that imply improving something in my life. So it can be with a GF, with a parent, my manager etc.. The argument "there is better or worse" does not take in consideration anything els that what I personnaly have.
Situation:
I would like to know what is the best answer to this argument in order to recenter the discussion on the main subject that should not be affected by what other does. I think that this argument is an easy trick to avoid any change and sometimes to just avoid a discussion.
Example #1
A : I think that we should improve [something].
B : Why do you want this ? You know, there is always better and worse, you should accept what we have now.
Example #2
A : I would like to have a better salary.
B : There is better and worse, you should accept what you have now.
Question
How can I counter the argument "there is better and worse" in order to make the interlocutor feels like he did not used a valid argument ?

A:

Your question can be re-framed as being overcoming resistance to change. 
In my experience reasons include:

The potential to be worse off as a result of the change
Fear of the unknown
Effort exceeding reward 
Ego - it is someone else's idea

When speaking about change, you should therefore frame the conversation around the benefits the change will bring, and describe the way in which the change will occur. 
Even ego can be overcome by seeding the idea in a conversation and then later referring to the conversation in terms of 'you know that idea we came up with last week'. 
In the example you gave you asked the wrong question - you told rather than sold.
An online search for 'overcoming resistance for change' will give you a wealth of information, although most of it is aimed at corporate change.

A:

There is a time for a lengthy, well thought out argument, and equally there are times when it can be unhelpful to use that approach. In controlled debates where two speakers are given an amount of time to speak freely without questions, a person may have the time to carefully dismantle an opponent's position and show it to be incorrect or unhelpful, and then make their own point in whatever manner they feel is best. However, in less controlled situations such as open meetings where anybody is free to interject and speak over you if they wish, taking too long to make your point can be disastrous. You may begin to make a very good point but get interrupted and criticised over an incomplete statement.
You're asking how to fight a clichéd argument, so naturally you probably don't want to resort to clichés yourself. Hopefully my suggestions below are not too clichéd, but my point is that a brief counter statement before you make any detailed argument is the best approach if you think that you may be interrupted.
I'm going to assume the example of a business context, where you have made an argument for improvement and somebody else is arguing to keep the business model as it is. The arguments could easily be adapted to other contexts.
Perhaps you could counter such a statement with:

True, there are worse models than ours; but that should not be a reason for not wanting to improve and become one of the best. If we are neither the worst nor the best then we are mediocre.

Or

Many of our competitors have programs of continuous improvement. If we do nothing, we will be left behind. If we are not moving forwards with everybody else, it is the same as going backwards.

Neither of these approaches directly attack the idea that "there are better and there are worse" or show that to be a fallacy, but rather they show that other external factors mean that refusing to change or improve is not viable.

A:

This is something you can say about anything. Anything can be better or worse. Pointing out that fact will make it obvious your interlocutor didn't use a real argument.
Some people use this kind of line precisely because they don't want to use an argument : they would rather not argue their disagreement. As such they simply use a blatant truth to close down on something you can agree with. It's up to you if you politely accept they dodge the debate, or if you feel they are getting away with it a bit too easily.
On the second case, you should try to dig their inner real reasons for disagreeing. So you can start by asking questions about their feeling about the matter.

A : I think that we should improve [something].
B : Why do you want this ? You know, there is always better and worse, you should accept what we have now.
A : There is always better and worse of anything. I thought my proposal was reasonable... what is the problem with improving [something] ?

