Ecumenism, the idea or uniting the whole Christian Church, might not be necessary or at least not the only way to go. Some thinkers regard it as an ideal to work towards.
Others are utterly determined never to let it happen. It may not actually be a good idea, given the limitations of human creatures.
The Bible talks about the unity of the believers. That's a good idea in itself. But it can only happen when there is real unity of belief, not by forcing agreement on some of the members of a church just for the sake of having one big group, instead of several smaller ones.
When I was new to Christianity the question bothered me a bit. Aren't the Christians supposed to be united. But a wise preacher put it this way: when there are several churches, you have to keep going back to the Bible to see what it actually says. In my case I needed to sort out the issue of baptism by immersion: was that what God actually called on His followers to do? Going over the question by looking at what is actually in the Scriptures meant that I gained a better insight than I had before. The different positions taken by different denominations mean that if you want to know about them, you check out what's actually written in the Book. It made sense to me then and it still does. When there was just one huge Christian church, it was possible for that church to tell its humble members, 'just do as you're told. Leave the thinking to the ones in charge'. In fact there was a time when ordinary people were not allowed to read the Bible themselves. Only the ordained priests were considered capable of understanding it properly. The result was that some serious errors or even deliberate malpractice crept into the picture. One particular Catholic monk saw the problem, blew the whistle on it and started the Reformation. Now the Bible is available in each person's own language, you can see for yourself and not have to rely on someone else to tell you.
It may be that different doctrines answer different needs, so long as they don't actually conflict with the Word. Some Christians I know find it important to be baptised by immersion. Others do not. Some like the formality used by the Anglican or Episcopal church. Others prefer not to have set prayers used in the service. Some prefer to kneel in prayer, others to stand. Some churches sing hymns, others only psalms. As far as I can see, none of those things conflict with worshipping in truth. They're just different forms of spiritual expression. God made us all individuals, and there can be different ways of doing what is still the right thing with the right outcome.
So we might not need one all-in super church. In fact we might be better off without one. Inter-church fellowship is important, but we can be friends without having to be housemates. I'll happily listen to anyone else's take on this. After all, we can all learn from each other.