Their argument is essentially that Canada has a lot of space (especially with climate change and all [2]), and by bringing in a lot of people from other countries, that will make Canada a more competitive country on the world stage.
In my personal opinion, I don't have anything against the movement, its just that all of these people are coming to either Toronto or Vancouver, and so those cities are becoming more and more competitive, meanwhile more conservative regions like Alberta and Quebec have a labour shortage.
Also, this might have a weird effect on French (I'm a french speaker so I might be a bit biased), but a lot of the people that are coming in right now (mainly from Eastern Europe, India, & China) only speak English, and not French. So we're seeing French go the same way as Scots or Gaelic did in the UK -- slowly dying.
Overall though, as long as we ensure that the population is distributed (I guess high housing prices are a good thing), and that people learn French, the 100 million thing isn't too bad.
--- [1] - https://www.bnnbloomberg.ca/canada-needs-to-get-to-100-milli...
[2] - https://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Ko... - Note right now, Only Toronto - Windsor has that light blue climate (same weather as New England), and in 2100 most of Canada will, while southern Ontario and BC will have the same weather as Washington or Normandy.