I think this is a good idea. See also recent changes to the en.wikipedia manual of style for mathematics formatting, https://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style/Mathematics#Using_LaTeX_markup, that recommended display=block over :-indentation.

Note however that this should only be done when the ONLY content of the indented line is a single math block. There will be some instances where a displayed formula drops out of math to add some additional text on the same line, and those will probably need a human editor to figure out what do do with them.

Maybe a maintenance template would be good for those so it is easy to find them.

I was about to write that there's a case where additional text on the same line can be handled automatically, namely when it's just punctuation. Putting a comma or a period outside the </math> tag is a fairly common typographical error which feels like it should be trivially fixable. But there are cases where that's wrong, like when the <math> tag ends with \end{align}. And there are cases which require editing the contents of the <math> tag, like when someone ended the formula with explicit whitespace like \, and \! to avoid HTML output. So really I think that this bot should:

1. Find lines starting with :, followed by any amount of whitespace, followed by <math>. (Yes, there are articles with whitespace there; I've never understood why.)
2. If there is a single <math> ... </math> block on that line, and if the line contains only whitespace after the </math>, convert the initial :<math> to <math display="block">
3. Otherwise, tag the line with a maintenance template.

Sounds good to me. Will this be applied to enwiki or to all wikis, i.e., there are quite some math uses in wikiversity.

In T268935#6654450, @Physikerwelt wrote:

Sounds good to me. Will this be applied to enwiki or to all wikis, i.e., there are quite some math uses in wikiversity.

I think it would make sense to do it everywhere +- local consensus.

I'm now running my script to look for awkward cases.

In https://en.wikipedia.org/wiki/Aristotle we have

:: <math>F=mv</math>,

with a double indent and a trailing comma.

In https://en.wikipedia.org/wiki/Arithmetic_mean

:<math>A=\frac{1}{n}\sum_{i=1}^n a_i=\frac{a_1+a_2+\cdots+a_n}{n}</math><ref>{{Cite web|last=Weisstein|first=Eric W.|title=Arithmetic Mean|url=https://mathworld.wolfram.com/ArithmeticMean.html|access-date=2020-08-21|website=mathworld.wolfram.com|language=en}}</ref>

we have complex ref on the same line after the formula.

https://en.wikipedia.org/wiki/Talk:M%C3%B6bius_function has various levels of indentation for the math formula with some quite deeply nested

:::<math>\sum_{k=1}^n \mu(k)=M(n).</math>

This is not just talk pages. https://en.wikipedia.org/wiki/Haar_measure has

* Left translate:
::<math> g S = \{g\cdot s\,:\,s \in S\}.</math>
* Right translate:
::<math> S g = \{s\cdot g\,:\,s \in S\}.</math>