i don't see that you can base a rounded political movement upon a single issue. put it this way, what is "greenism", as a ideology along side conservatism, socialism, liberalism etc. i see rehashed socialist policies, but their ultimate aims are inconsistent with the industrial world that socialism is very happy to maintain (but change ownership and control of). its a bit like having the Health party or the Education party, UK Independence party, a big focus on one issue but otherwise stealing others clothes.
when i see them up along side the other parties they dont seem to fit. maybe its their leader, while i have sympathies with the other party's objectives (even if i disagree with them or their method), i don't for the greens. all parties accept a good slice of green policy as default, like providing a health service or education is seen as a given now ( the debates are over delivery).
I think it all depends on the political system. In our current FPTP system, in the event that a majority government is elected, I agree with you. However, in a minority/coalition government I think it's completely feasible to have single issue parties, if that single issue is deemed important enough by the electorate and that the electorate feel that none of the major parties place anywhere near enough importance on that issue. This is the case with UKIP and immigration - it's supporters clearly don't feel that any of the major parties place immigration highly enough on their agenda. I'm sure many UKIP supporters would not necessarily want a UKIP majority, but they may want a representation of UKIP in government, therefore bringing immigration to the forefront of discussion.
It's the same for many Green voters and climate change.