Well maybe think about it this way, Na'Vi may be cautious of paying lots of money for a young up and coming awper. I can't think of a great awper that is available currently, that would be worth risking the purchase for, without knowing it will work.
First I believe Na'Vi want to see what they've got with Boombl4 being the IGL and acquiring an awper, maybe then if GuardiaN doesn't perfect but the team is looking good then they swap him. I can't imagine FaZe will be asking for a lot for GuardiaN because they seem to want a lineup change and GuardiaN's name has been thrown out there to get kicked for a while. At the same time, GuardiaN also has a lot of experience in Na'Vi and therefore they already know what the chemistry will be like.
I believe this is a good and safe choice for Na'Vi, and if GuardiaN can focus solely on awp and perform, then I believe he can be more valuable than awpers that are considered better right now anyway.