#1. Sampras was retired and Federer was chasing a fixed target whilst being in his absolute prime. So it was expected that the rate at which Federer was racking up slams, he would one day overtake Pete. And FYI, Fed was proclaimed GOAT only after he overtook Sampras. Only some fanatics and fan-boys crowned him before he got to 15. Most of the tennis/RF fans waited till he got to 15 to claim GOAThood, which was normal.
#2 Federer is still a moving target. He is not done yet. He is slower and more prone to upsets, definitely. But till he plays, he remains a moving target, unlike Sampras. He will always have a fair shot at Wimbledon.
#3 Nadal has not been able to beat Fedovic off clay for the last 5 years. The streak is now 14-0. So if he meets one of these 2 outside FO(or even Medvedev for that matter), the odds are stacked against him. He has not figured them out outside clay
#4 Nadal had quite a subpar clay season in 2019, by his standards. His level was far far from his prime and he just about found another gear in time for the French Open. Luckily from him, there is only Thiem posing a challenge on that surface and he hasn't shown enough gumption to take down Nadal. But the cracks were there to be seen in 2019. He isn't getting any younger next year.
#5 New gen is finally getting their act together. Shanghai is a sign of things to come. Medvedev having the hottest streak by a youngster since forever. Tsitsipas coming into his own this year beating Federer and Djokovic on hard and Nadal on clay. Zverev showing signs of potential. Many other youngsters who could break new gorund in 2020 like Shapo and FAA.
The situation is a lot more complicated than it looks. Yes, there are few things more certain in life than Rafa winning RG, but hold your horses. Don't count the chickens before they hatch because neither is Fed done, nor is Rafa in his absolute prime to be making over-confident predictions.