I'm frankly confused about why UTF-16 and UTF-32 etc. exist at all

UTF-16 exists because Unicode was originally supposed to be a fixed-width 16-bit encoding and many systems were designed during this era and needed to be retrofitted to support more characters. These aren't some niche systems or systems that are on their way out, they are major current techologies like windows, .net, Java and QT.

UTF-32 exists because some people think it's easier if each code point is stored in a fixed-size unit. IMO this is largely illusory as there is not a 1:1 mapping between unicode code points and what users would call characters (for example most users would say that "Spın̈al Tap" has 10 characters but it requires 10 unicode codepoints to represent) but it nevertheless exists as a perceived advantage.

I don't think either of these encodings is going to go away any time soon. So if your system interacts widely with other stuff you are likely to end up dealing with other unicode encodings sooner or later.

Will UTF-8 ever become "obsolete" and replaced by "UniversalCode-128" or something, which also includes the alphabets of later discovered nearby galaxies' civilizations?

If we ever establish meaningful contact with intelligent alien life then some decisions would need to be made as to how to represent their languages on our computers and vice-versa. That could eventually mean switching away from computing standards as we know them today to a new set of interplanetary standards.

I think realistically though it's incredibly unlikely that will happen. IMO even if alien-life exists and even if we discover it, it would be impractical to establish meaningful communications without faster than light communications and/or travel and that would mean breaking physics as we know it.

Assuming we don't establish contact with aliens and assuming that we keep using computers that resemble those we use today, it seems unlikely that our text representation systems will be radically changed, it's possible that at some point the codepoint space will be expanded, but I think it's more likely that greater use will be made of combining characters, variant selectors etc to allow new languages to be represented with fewer code-point allocations.

I'm frankly confused about why UTF-16 and UTF-32 etc. exist at all

UTF-16 exists because Unicode was originally supposed to be a fixed-width 16-bit encoding and many systems were designed during this era and needed to be retrofitted to support more characters. These aren't some niche systems or systems that are on their way out, they are major current techologies like windows, .net, Java and QT.

UTF-32 exists because some people think it's easier if each code point is stored in a fixed-size unit. IMO this is largely illusory as there is not a 1:1 mapping between unicode code points and what users would call characters (for example most users would say that "Spın̈al Tap" has 10 characters but it requires 11 unicode codepoints to represent) but it nevertheless exists as a perceived advantage.

I don't think either of these encodings is going to go away any time soon. So if your system interacts widely with other stuff you are likely to end up dealing with other unicode encodings sooner or later.

Will UTF-8 ever become "obsolete" and replaced by "UniversalCode-128" or something, which also includes the alphabets of later discovered nearby galaxies' civilizations?

If we ever establish meaningful contact with intelligent alien life then some decisions would need to be made as to how to represent their languages on our computers and vice-versa. That could eventually mean switching away from computing standards as we know them today to a new set of interplanetary standards.

I think realistically though it's incredibly unlikely that will happen. IMO even if alien-life exists and even if we discover it, it would be impractical to establish meaningful communications without faster than light communications and/or travel and that would mean breaking physics as we know it.

Assuming we don't establish contact with aliens and assuming that we keep using computers that resemble those we use today, it seems unlikely that our text representation systems will be radically changed, it's possible that at some point the codepoint space will be expanded, but I think it's more likely that greater use will be made of combining characters, variant selectors etc to allow new languages to be represented with fewer code-point allocations.