You're entirely right. The assimilation rate in Alberta is 75%. That means that, if I'm francophone and I have four children, only one of them will remain francophone. Exogamy is probably one of the major reasons for this situation.
However, from a relative standpoint, the figures are declining. Mr. Castonguay, a researcher from Quebec, often emphasizes relative rates. However, you also have to understand that, in absolute figures, the number of francophones has increased in Alberta.