How we change what others think, feel, believe and do
If they have any negative elements to their argument, attack this aspect, positioning any negativity as bad and inadmissible. Make any negativity so bad that even a small amount allows you to throw out their entire argument.
Say that they are being pessimistic and only seeing things in a negative light. Highlight their unwillingness to take even simple risks. Frame them as critical and uncaring. Indicate that they have anti-social values.
That's a rather negative way of looking at things.
So you think we've failed in some way. Let me tell you how we have succeeded...
I wish you'd stop criticizing and start being constructive. We have to get this done and just knocking things isn't helping.
Are you implying that what I have now is bad? Sorry but you just can't say such things.
The need for change is often created by attacking the status quo, asserting that 'we can't stay here'. Any such attack can be counter-attacked as being negative.
Another ways of attacking the status quo is to promote a better alternative. Even this may be repelled by highlighting how it implicitly attacks the current arrangements.
In some business cultures there is a strong 'can do' attitude and negative viewpoints are easily framed as being automatically bad and wrong. This is also true of general social situations where it is easier to fend off pessimists as being negative and unhelpful.
A paradox about this defense is that it is being negative about being negative and so commits a sin against itself.