The big problem is it is not enough to define as a centre, because normal non political human beings don't really care where you fall on some imaginary political line and will happily vote for just about anybody on the "spectrum". You cannot win (or maintain success for very long) when you only define yourself politically in positions relative to other people, because it means you just move with the wind. Especially as what most pundits view as sensible centrism is very different from what most ordinary people view as such.
I think you are making a lot of assumptions that are unfounded, but this paragraph is the most revealing in more than a few ways:
2. Most of us have busy lives so those closest to the centre aren't active politically while those furthest from it are most angry at things so more motivated to get involved thus when selecting the leader of a party or choosing policies, those furthest from the centre (left in case of centre-left parties, right in case of centre-right parties) are more likely to show up and get involved while also the nosiest so representatives may wrongly assume they are larger than they are.
One of the big problems with politics is is genuinely quite difficult to figure out how representative a party's cadre is of its electoral base. This is not a hugely new phenomena, so it can't be blamed for any "hollowing out of the centre" but it does lead to some of the more brutal factional spats as both groups view themselve as the more accurate representation of the base.
However this paragraph (in the subtext) is quite damning of the very centre that you hope to bring back. You seem to claim that the defining feature of the centre is a lack of emotion and passion, qualities which apparantly exist in abundance in the fringes. If this is true (it isn't - many of the most passionate people around are centrists, and many fringe people are dull wheezebags) then it would be a stunning indictment of the political centre, because you need enough passion to inspire cadre to come out and fill seats. And I think it's a lie to suggest that you cannot inspire people without being on one extreme or another; you just have to truly believe in something which isn't defining yourself as a central point to two other points.