Let's not jump the gun here. How will Nadal win multiple slams next year? He hasn't even won the UsOpen this year yet.
Plus he has not beaten Federer in a big slam match. The last 2 matches against Fed were pretty one sided. Who is to say he will beat Fed next year in a big slam match?
And then of course if Djokovic comes back swinging , he will be a big thorn for Nadal. Not to mention Kygrios, Zverev, Dimitrov etc etc.
Then again Nadal might get injured. Especially if he over plays at the end of this year for #1. How many years has Nadal had completely injury free?
How can we assume 2018 will be like 2017 for Rafa? He has Never had 2 consecutive years at the top.
The only thing that can be said is that Nadal has good chances at the French Open next year. If, and it's a big if, he remains healthy.
Fed's slam record is safe. Even if Nadal wins more , it's not like Fed won't win more as well. Fed has a great chance in several Wimbledons going forward.
Nadal is playing for 2nd place, Fed is already the GOAT, and I doubt Rafa loses sleep over it.