Thank you for the opportunity to help you with your question!
Once crossing over doesn't happen, genetic variation is being limited to only independent segregation, random assortment, and random fertilization. Also, without crossing over, the composition of chromosomes will never change. the consequences:
1. Genetic variation among individuals of future generations will be largely restricted. For say these 2 genes are linked. One bad, one good. If the bad trait results in selective disadvantage, the same risk of removal of the gene from the population is also on the good gene.
2. Being genetically similar, any bottleneck effect, for example, could potentially wipe out a lot more numbers in the population. This could pose a higher risk to extinction of the population.
Please let me know if you need any clarification. I'm always happy to answer your questions.