我觉得这里不宜把 their life 换成 their lives。

因为这里的 their 表面上是复数代词，但实际上是表示单数意义——它指代的是 a person（单数名词），这里之所以用复数代词，是因为 a person 本身不显示性别，如果后面用 his 或 her，无法满足 a person 的性别模糊性。

不过，按传统语法，人们会用 his，即：

Life if precious. … To a person nothing is more precious than his life.

但这样的用法后来受到女权运动者的反对，所以后来为了平衡这种性别选择（避免性别歧视），人们就用复数代词来指代性别不明的单数名词。又如：

One of the students has left their book behind. 有个学生遗忘了一本书。

Everyone has their own way of doing things. 每个人都有自己的做事方式。

Anyone in their right mind would find it unjust. 任何思维健全的人都会觉得这不公平。

Each person reacts to their environment differently. 每个人对环境的反应是不同的。

Everyone is free to express their own opinion. 人人都可以自由表达自己的意见。

Each student will have their own coursework folder. 每个学生都要有自己的课程作业文件夹。

Se heard someone blow their nose loudly. 她听到有人大声地擤鼻涕。

If anyone calls, ask for their number so I can call them back. 如有人打电话来，问问电话号码，这样我可以回电话。

Every member will receive their own 'Welcome to Labour' brochure. 每个党员都将收到一本名为《欢迎加入工党》的小册子。

But anyone looking for income from their investments is in a much worse state. 但是，那些指望靠投资获取收入的人境况更为糟糕。