How easy is it to import existing project data into these alternative tools?
Although it depends from tool to tool, the majority of Microsoft Project's alternatives will have a way to import .mpp files into their tool. And, if you think about it, it makes sense. Microsoft's tool was the top project management software for at least two decades decades.
Because of it, a lot of big corporations (potential clients for these alternatives):
Work with other companies that use this software
Need this feature to consider migrating to another tool