While 6F's analysis makes sense to organisations that think this way - unfortunately the 'people' budget and the 'travel' budget are often kept very separate and aren't equated.

The person is already paid for but travel becomes further operational expenditure.

Since productivity can rarely be shown to be different easily - particularly at the senior levels where they are likely to be working all day and night regardless - the end result is that travel planners simply restrict travel to Y and say 'be done with it'.

My place has WT+ allowance on LH. Thankfully our CEO likes it - he often pays for AUPs to get him into business and that'd become much more costly from discounted Y. But finance have threatened removal several times regardless of productivity arguments.