Nuance of the structure "Verb (stem of negative form) + ざるを得ない" compared to "Verb + しかない"


I've browsed a bit throught the other posts and it seems like there is no
clear explanation of the nuance of "V + ざるを得ない" compared to "V + しかない".
Roughly speaking, they have the same meaning but is there any subtle nuance I
don't get here or are they completely interchangeable?




(せ)ざるを得ない looks a little more literary, stiff and formal, although it can
safely be used in informal but serious situations. It clearly expresses that
the verb is an **undesirable** but unavoidable option. In other words, you do
not want to do it but cannot help it. ざる is an archaic negation marker, and 得る
has a potential meaning. ざるを得ない is one of the [set phrases that use archaic
forms](https://japanese.stackexchange.com/a/42724/5010).

On the other hand, (する)しかない is fairly neutral ("one can only ～"). It's usually
interchangeable with ざるを得ない, but it does not necessarily imply the verb is an
undesirable option. For example, you can safely use しかない when you noticed you
had only one option but it was your initial plan, anyway.

>   * 戦わざるを得ない！ (strongly implies you are reluctant but had no choice)
>   * 戦うしかない！ (more proactive, "I must fight!")
>




(せ)ざるを得ない looks a little more literary, stiff and formal, although it can
safely be used in informal but serious situations. It clearly expresses that
the verb is an **undesirable** but unavoidable option. In other words, you do
not want to do it but cannot help it. ざる is an archaic negation marker, and 得る
has a potential meaning. ざるを得ない is one of the [set phrases that use archaic
forms](https://japanese.stackexchange.com/a/42724/5010).

On the other hand, (する)しかない is fairly neutral ("one can only ～"). It's usually
interchangeable with ざるを得ない, but it does not necessarily imply the verb is an
undesirable option. For example, you can safely use しかない when you noticed you
had only one option but it was your initial plan, anyway.

>   * 戦わざるを得ない！ (strongly implies you are reluctant but had no choice)
>   * 戦うしかない！ (more proactive, "I must fight!")
>

