There's no doubt that if immigration remains at the historically high level that it is at today, that fear is entirely justified.
Today Quebec welcomes 45,000 new immigrants each year (that limit will be increased to 55,000 in 2010), Since 1995, according to the Quebec government statistics, the province welcomed 598,000 new immigrants. Subtracting from that number those immigrants who leave to greener pastures in BC or Ontario, the net effect is that Quebec has gained 480,000 immigrants since the last referendum.
In that 1995 referendum, immigrants as a group voted over 90% against the sovereignty option and were largely blamed for the referendum defeat. (Remember Jacques Parizeau's "Money and the ethnic vote" remark.)
Today Quebec does a better job of absorbing newcomers into the French side of the language divide, so it's fair to say that in any new referendum the number of immigrants voting for sovereignty would be higher. That being said it would be an achievement if the number reached 20%. Even if we use this optimistic (or pessimistic) higher number in a hypothetical scenario, it augers badly for the sovereignists.
Let us consider the razor thin margin that the "NO" side won in the last referendum. Out of the 4.7 million votes cast, the NO side received 2,377,028 votes (50.58%) and the YES side received 2,322,740 votes (49.42%.) Just 54, 288 votes (1.16%) separated the Yes from the No's!
Let us also imagine another referendum, one held today in 2010, where everybody voted exactly the same way that they did in the 1995 referendum. The only difference now, we will incorporate fifteen years worth of new immigrants who will vote 80% in favour of the NO side. Under this scenario, the NO side would get an additional 358,000 votes and the YES side an additional 120,000 votes, for a net gain of 238,000 votes for the NO side.
The new totals would look something like this;
NO 2,735,028 (52.9%)
YES 2,442,740 (47.1%)
Difference - 292,288 (5.8%) versus 54, 288 in 1995.(1.2%)
For sovereignists, it is a frightening scenario. If we project things out a little farther it even gets worse (or better)
If Quebec maintains the historically high figure of 55,000 immigrants per year, it will mean that in five years, there will be another 225,000 new ethnic voters and the results of a referendum in 2015 might look something like this, again, using 1995 referendum numbers as a base.
NO 2,915,028 (54.0%)
YES 2,487,740 (46.0%)
Five more years of immigration will yield:
NO 3,095,028 (55.0%)
YES 2,504,740 (45.0%)
If things trend out, twenty-five years after the referendum of 1995 (10 years from today), immigration will have added 744,000 votes to the NO side and only 186,000 to the YES side. Immigration will represent an absolute swing of 10% over to the NO side as compared with the 1995 referendum.
It means that in 2020, 5 out of every 7 seven francophone Quebeckers will need to vote YES in order to achieve sovereignty. Not likely......