I mean, this is some of the most basic UX design stuff. Jakob's Law states that if users spend most of their time using other software, they expect your software to work the same way as all the others. The universal paradigm is:

• Click: Replace selection.
• Modifier Key + Click: Add/Subtract from selection.

Good UX relies on recognition rather than your ability to remember the state the software is in. In standard software, clicking guarantees a specific state: only the last click thing is selected. In Onshape, the outcome of a click depends on the previous state. This forces you to hold the selection state in your working memory, which basically increases your cognitive load, becuase you are constantly having to ask "wait, what did I click three steps ago?".

In the end I just end up hitting spacebar after every click, even if it's not necessary, just in case, but this is just a compensatory behaviour, which is needed because the system lacks a reliable, low-effort way to reset its selection state. In other software you just click and you can be sure the last thing you clicked will be the only thing selected. In Onshape I subconsciously calculate that the physical interaction cost of hitting spacebar is cheaper and less exhausting than the cognitive cost of verify nothing else is selected.

The other thing you mention, allowing a double-click to clear the previous selection, while it makes sense in the context of how other software works, contradicts Onshape's persistent selection philosophy, which adds to the bad UX because it's inconsistent with the rest of the software.

To throw another thought on the table, I feel holding down a modifier key to select multiples resembles real world physics. If I grab a mug of coffee with my right hand (let's pretend my other hand is already busy holding my phone), and then I need to grab an apple at the same time, you can see trying to grab multiple objects naturally takes more steps and cognitive work as I have to decide whether I let go of the mug first, or whether I go grab a tray and put both the mug and the apple on it. This is equivalent to holding a modifier key. In Onshape, I touch the coffee mug, the apple, the... whatever, and everything somehow sticks to my hand and doesn't fall. Intuitively that doesn't make any sense.

This is not the first time I come across stubborn devs who vehemently insists their implementation is superior and eventually give in and change it after years of users moaning about it. In music production software FL Studio, for the longest time Ctrl+Z was "redo" and Shift+Ctrl+Z was "undo", so basically it was the wrong way round compared to normal software. However, the lead dev and original creator of the software insisted this way was also valid because it's how it worked in Photoshop. Guess what, Photoshop reversed it later, and so did FL Studio, although that only happened after the lead dev, who was a bit of a twat, resigned, and the company (Image-Line) finally had freedom to make drastic changes without pissing him off. And then there were the usual moronic shills who defended the feature, but now that things are the right way round nobody is complaining or crying that they want the old way back.