Western Europe is in the receiving end of the OBOR. So far, there have been more than 39 direct rail links established with Western Europe via Central Asia and Russia. Hence, with the top leadership joining or not, Europe is in the game. It is almost half of the game.
The EU salivates over the OBOR, what they are concerned about is the potential dividing effect of the OBOR if China decides to negotiate the member states individually.
If you really want to see the level of coordination, check out the cooperation between the AIIB and EBRD.
For those with brains the size of a chicken, of course, a political show of high-leaders talking big would mean everything. But, for those who really know how this thing works, what is going on behind the scene among the bureaucracy/experts is what really matters.
If China is doing trade with Spain, France, Germany, Holland, Denmark, England via rail lines, this means POLITICAL WILL that actually results in concrete action on both sides.
Please stop being so simplistic of counting the number of heads of states. Silk Road conference is almost like a forum, hence, it would be inconceivable every single of them joining. Besides, most European leaders are their hands all full before they worry about coming to the forum and delivering a speech there.
Nonetheless, all of the developed European countries are to be represented at quite high level, if not at the highest.
US, Canada or Australia not joining (for the genius above) does not rally matter (it would be funny if Trump joined, in fact) because the Silk Road has NOTHING to do with North America or Australia. Silk Road is all about Eurasia.