Alliances are essential to get what is arguably the best trade route you can get: International using the Wisselbanken policy + Democracy + Reform the coinage (golden age dedication). which give you the best of both worlds: food/production like a domestic route and more gold than you can expend. It's not only impactful, it's the cornerstone of my strategy. My last game I played as Norway, so I didn't try to get alliances because I wanted to pillage the jesus out of everyone, it was just sad to have to choose between gold and food/production when I could have plenty of both if I had an alliance, plus a bit of other yields.
Some Alliance bonuses aren't impactful but there still stuff that can make a huge difference in specific victories. A tier 2 religious alliance give your religious units +10 strength, which is extremely good for a religious victory. If you can keep a military alliance while going for domination, inviting your ally to your wars for that +5 strength is quite useful and on tier 2 you get 15% production for units. Cultural alliances are good for cultural and to a less extent scientific, because of the tier 2's extra great people points. Alliances are essential for Diplomatic victory because of favors, being the victory where it have the most impact.
I was trying to write a complaint but I kept disagreeing with myself and calling BS on what I wrote

, so I can only conclude that I actually like where diplomacy is now. If you're peaceful it's too easy to keep everyone friendly and the only thing that can make the AI say no when you ask to renew a friendship is grievances but honestly, I actually like having alliances that last forever. IMO diplomacy is in a pretty good place now. It can improve but we had worse, much worse.