たら is a grammar structure which means if...then.
But what does たら mean in this sentence?
この権益の擁護には真剣たらざるを得ない
The sentence means
We must be serious about defending this interest
What is the role of たら in this sentence?
It seems that it does not mean if...then here.
What you see is not the conditional -たら, but -たる + ざるを得ない.
X たる ("be X; being X")
This form is a vestige of an old sibling of what is now called na-adjective, which still occasionally appears in formal speech. It stands particularly for state that is acquired (instead of inherent), temporary (↔ permanent), apparent (↔ internal), or due (↔ actually is).
In spite what some dictionaries say, it does conjugate in the same manner as verbs なる or おわる (consonant-stem; type I) when needed, though most frequently seen in the dictionary form as an attributive to a noun.
ざるを得ない ("have no choice but")
A fixed phrase to learn by rote, but consists of ざる (archaic "not -ing") + を (accusative) + 得る ("be able") + ない ("not"). Since it starts with a negative, words that come before it must be changed into negative form:
行かざるを得ない
見ざるを得ない
同意せざるを得ない (⚠ suru verbs have an irregular form!)
and
真剣たらざるを得ない cannot but assume a serious attitude
