Newcastle's performance was really good at first; I even thought they would win the match, just the way you think, but at the end Liverpool showed them that they are still good and can pull up at any moment. Although you are right, if Liverpool were a weak team, Newcastle would beat them in that match, but they are not. This shows that every club is preparing for this season. It will be an interesting season because many clubs will fight for the title this season. The league just got started; let's see how it will go.
Liverpool conceded two goals after taking the lead because of to a weak defending, if Liverpool defense was strong enough to prevent Newcastle from scoring goals, I doubt the home team would be able to equalize both of the goals. Liverpool attack is very strong, but their defense is not good, which is going to affect them, as we noticed by their first league game against Bournemouth, they struggled to win since they conceded unnecessary goals everytime they scored.
The Premier League will be difficult to predict right now because the season is only beginning, and we don't know which team will continue to win. I do not expect anything from Manchester City this season, they've remained unchanged despite spending a lot of money to buy players. Perhaps Liverpool's next game against Arsenal will determine which side has the best chance of winning the Premier League title this season.