A lot of the 'No' voters seem to think that if we vote 'Yes' on AV that we'll be stuck with it for years, with no opportunity in the future for PR...
However, as far as I'm aware, it's only the fact that the LibDems got a slight bit of power in the last election that we're seeing any electoral reforms at all, and if they'd won the election outright we'd now be looking at a referendum for PR. The majority of Tories and Labour would never initiate electoral reform as it's hardly in their interest, so unless the LibDems can keep on getting enough power to form a coalition, this may be the only chance of electoral reforms we're going to get.
And why is it the LibDems want AV? As we all know, there are plenty of constituencies where voting for the LibDems is seen as a wasted vote, and therefore people will vote either Tory or Labour purely with the intention of keeping the other party out. Under AV, other candidates will actually have a chance in these consituencies as people will vote for who they actually want to win, and I think as a result it's likely many more LibDems will find themselves in power.
With more LibDems in power, they'll keep pushing for PR as it's in their interest as shown by the previous election (something like 23% of the vote vs 9% of the seats), and it's also in the interest of the country as a whole in my opinion.
If we keep the status quo, how likely is it to happen again that Tories or Labour will need to concede electoral reform to the LibDems to form a coalition? Not very.
How likely is it Tories or Labour will support electoral reform if they get in on a majority? Not very.
If we want to see changes in the future, we can't expect it to happen under the current system. We need AV.