Is the left's defence and support of Islam in Western society it's downfall?

I suppose the left would argue that: no, it's not support of Islam, but rather support of universal human rights.

To which the right would say that by offering those universal rights, along with a comfortable space for Islam to exist and flourish over time erodes the dominant culture, as it needs to become "welcoming" and accommodating of foreign ideas, and even tolerant of ideas that are completely counter to the existing culture. This accommodation does not exist in Islamic countries, many of which require some adherence to local custom and culture from visitors.

I have a feeling that despite whatever rationalisation you may have for the continued global rise of the right, it is due to most people prioritising this issue (whether vocally or silently) over others, even over economic positions that may benefit them over the longer term. It is the most pressing, and other things can be dealt with after.

If Islam specifically were to be addressed, I think we'd see significantly fewer conversations about multiculturalism or immigration.

It does however seem that the left is happy to die on this hill, which is utterly frustrating, as it alienates so many people that would otherwise form the core support base.