Twitter likely feel that it's easier to monitise users if they're using official clients. The API and the early developers have served their purpose in attracting enough users, now they're more of a hindrance so time to cut them off.
I'm not saying I agree with this policy (I absolutely don't!).
WHy don't they "just" serve ads using the API as well? And ad a line in the EULA saying something like: "Not showing these ads will mean termination of your Developer Account" or something like that
Looking at the product as it is now, offering an API for third party app developers that clones the primary timeline browsing experience is a no-no because Twitter gives up control over how they can craft their product experience. For example, look at their official app's Discover tab. They can't offer something like that and iterate quickly easily across all apps.
Serving ads through the API alone also limits ads delivery. For eg., they might want to display full screen video ads like how iAds does it. Or they even want to do something even more fancy (remember the dick bar? Imagine if there was no third party Twitter apps when they launched it). Without complete control, it's hard.
In a way, they are at stage 2 of their evolution, stage 1 was iterating on their core product, now they need to iterate on their business model.
PS: I'm the developer for SimplyTweet. I'm biased, but I can imagine where they might be coming from, even though I'm not happy about it.
How much of a factor were the client limit and the API changes? I'd like to better understand if this is just you moving on or if twitter has made it impossible to continue your app.
Not impossible, but it's highly improbable to continue. The API changes aren't the biggest factor, but it requires some changes across the entire app, and I would like to take the chance to revamp a good part of it (It's major version 3, I've revamped it twice across the years already). Till now, they haven't answered me on how many user tokens I have used up, so it's a big risk for me to put in major effort, not knowing how much sales I can get in return. There's also some technical issue users are facing that they have been very slow to respond to (read: not yet), so frankly, I think I'm a low priority to them anyway.
I remember when I was on honeymoon and my web host fired me an email about blocking internet access because I was overloading their DNS server. I explained my situation (hey it's a production site and I'm away on vacation) and requested for them to keep me up for 4 more days when I'll be back home. They refused and I had to worked on it at the hotel, while my newly-wedded wife waited. I remember when I woke up in the night 2-5am, doing support, replying to users. I was fanatic about supporting users well.
I loved what I did. But that love has waned. Why stay when you aren't welcome?
It's a shame. SimplyTweet was the only twitter client I've found that lets me digest my twitter feed the way I want to. Now I need to look for an alternative. And presumably repeat the process until all we're left with is the standard client.
I'm not saying I agree with this policy (I absolutely don't!).