I've gone from a 450D to a 50D to a 5DII and last year I had a difficult decision to make. I pretty much only take photos during holidays so we try to go on holiday as often as possible so I doubted if I would really need a DSLR. In short "downgrading" to a mirrorless turned out more expensive than I thought and really not that beneficiary to me personally. Going all the way down to a "pro" level compact was tempting, but I was afraid to miss doing real photography and not be happy with the quality of the results. Then I chose to upgrade to a 5D3, however in the last couple minutes of decision making I saw the 5DS for a relatively small added cost...so I went for the 5DS.
I love the camera and it's so much better on pretty much every level compared to the 5D2 that I think I'm set for a while and don't need an upgrade soon. However the 5DS turned out to be quite an expensive upgrade as soon as I downloaded the first files to my computer(which was quite a good computer)....I needed a new pc badly with the large files. So I did a big overall upgrade.
I had some doubts if this was all worth it considering the amount of photos that I take, but it's also a hobby and hobbies just cost money.
Since then I also invested in a Lee Filter set and I'm more and more enjoying to take photos again so in the end I'm happier than ever.
Do I miss certain things? I'm a simple guy and use probably only 5% of the features that my camera offers(same counts for the little amount of apps on my phone). But one thing I do envy others for is the swivel screen when I have to go into backbreaking positions to take certain photos. I think the screen would be something I'd upgrade for in the future.