None of us will be around forever. Some of our software might be...
That's the point of the exercise: to identify mission critical people and make them not critical to the entire business. If your business hinges on one person who knows everything and works on everything, you're fucked if they disappear and you haven't ensured at least some redundancy.
>Most work [during someone's vacation] will just be queued up until the person comes back or worse the person will be contacted during vacation to answer questions, provide information or do some work.
Nothing in that sentence implies that the person is referring exclusively to an especially important position.
As a senior eng, I try to do the same when I have time off - work notifications are muted, and my away message is typically set to "I'm away for X days. If you to reach me, no, you don't."
1 week, people can mostly ignore your job function and limp by
2 weeks, people can do ~50% of your job and limp by
3 weeks, you do need someone to complete all/most of your job functions.
Do you feel guilty when they pay you too? Paid vacation is a form of compensation.
Every day of your ~30 days of vacation which you do not use, they likely have to pay you out as a full day's worth of wages anyways.
Most will prefer you take a month or two each year off, recuperate, and come back refreshed and productive, rather than having to deal with the bookkeeping and tax hassle of paying you extra.