simplify

Hello

- instead of making new features every month, polish the existing ones. Make performance improvements.

Which screen or feature are you having performance issues on? Could we have the information that would allow us to see where the problem is?! Messages? Application response time? Size of your servers?

- remove the tension of unnecessary themes

Currently, there are only 2 colors to choose from with a single display, could you be more specific on what you call a theme?

different types of views.

We adapt to our users and as no one has the same habits on how they work, we offer the possibility, indeed, of displaying the screens in different ways. But, once you have chosen the way to display, there is nothing forcing you to go back to it, so what are you talking about when exactly?

Make just one that is actually good,

Which would be best for you? Would the one you like be the same for hundreds of other users? Do you have any concrete examples of what should be good or not? Cravings ? Have you already shared this on the forum? What message ? When ?
Be as precise as possible so that the specific requirements of our users can be taken into account!

and doesn't have basic margin/padding errors.

Can you share with us screenshots of this margin problem? So that we can resolve the problem quickly, as we try to do every time!?

Thanks for your comment

- instead of making new features every month, polish the existing ones.

That is exactly what we did on V11.0 : improve existing (Gantt, Today, Start guide)

Make performance improvements.

We really focus on performance.
Every time a performance issue is pointed out, we fix it.
Our lists can display thouthands of lines without performance issue.
I'm not sure competitors can say the same.
It you face performance issues, please tell us
NB : we won't focus on performance issues concerning old versions ( ) and misconfiguration of self hosted servers.
... I saw so many performance issues due to bad configuration of PHP and MySql, or undersizing of server (hundreds of users connected a 2 cores server with only 4GB Ram)

- remove the tens of unnecessary themesand different type of views.

It is what we did From V9.0 with new Gui. Single interfarce where you can just change colors, not really theme).
So we don't have themes any more, unless you use a very old version, or use the Old Interface (users can still switch back to old interface if they prefer it, but we'll remove it one day, on a future version)

Make just one that is actually good, and doesn't have basic margin/padding errors.

What are these margin/padding errors ?

What version are you using ?
Your remarks seems to be several years old.

- when i was looking through the securityGetAccessRightYesNo function in the source code, to check what permission might be needed to see attachments, I found it to be completely unreadable. And lines like this: $accessRight="NO"; // TODO : why is it no here ? don't exactly inspire confidence. Do you have tests for it?

Well, you're righ, the code for access rights part is quite complex to understand.
Not even everyone in the team is able to update this part, as it is, yes, very complex. Possibly because management of rights in projeqtor is so adaptable...
You may be right, the complexity may come from a technical debt, as it incrementally increased in complexity, but as of today ot works and is reliable as been highly tested by users. Not sure we would be able to rebuild this part from start and get something as reliable in all cases.

- resource planning is slow on the javascript side - probably needs some virtual scrolling or other trickery.

Yes, Gantt chart may become slow with many lines.
Virtual scrolling may be an alternative. It is what exists in Lists (Activities, Projects, ...).
But technically is will be quite complex to implement in the Gantt chart context, with two parts : the tabular part and the graphical part, with bars.
It would be relatively easy for Table part, but would be much more complex for the graphical part, with constraint to preserve the two ways scrolling possibility for the two areas, while always keeping bars facing corresponding lines.
So yes, idea is great, developping it will be a real challenge (and a high cost for us).

- then remove the old gui. It is just causing confusion when every user has different interface. And the source code of if old {} else {} does not exactly help you either.

You're right hte old Gui is a real mess.
Even for us, it's a thorn in the side.
We plan to get rid of it.
But it seems some users still use the Old Gui, to preserve their habit or because they find it more efficient.
So we'll remove it in few version...

- there are inconsistencies even with the new gui on basically every screen. Here is an example from two random screens

Eagle eye !
Well, if it's all what bothers you, it shouldn't keep you from sleeping.

- the whole wanna-be single page application approach with the arbitrary limitation of one tab open and login session (constant logouts) feels quite buggy. And why can't i have different tickets on different tabs open - feels unintuitive.

uh , uh 
RTFM
Did you see the new tab icon ? Or try CTRL+clic on any ticket in the list ? Or CTRL+clic on any reference, like linked elements ?
(Marylène who manages the Online User Manual gets very angry now, so I'll feed her with candy to calm her down)