For us it is. In 99 years when we're gone, we'll wish we had kept it.

That's the rub, isn't it? If the UK doesn't cut this deal, and decides instead to maintain a hostile and antagonistic position towards Mauritius, can they keep it 100 years? The track record on that isn't great. Again, see Panama. A 99 year lease at least locks it in for 99 years; having to maintain an unpopular and somewhat indefensible assertion of colonial sovereignty in the modern day seems far less secure, IMHO.

Plus, the average elevation of Diego Garcia island is (checks internet) only 4 feet:

installations.militaryonesource.mil - Navy support facility diego garcia

Not sure how much national security value we should assign to holding that spot 100 years from now.

National security trumps Feelz at the UN 100/100 times.

Except that feelz at the UN are a part of national security. China is an economic and diplomatic threat, not just a military one. They're using the Belt and Road project to wrap up allies and gain influence in developing nations all over the world.

The UK's national security is probably better served by having 99 years and removing the diplomatic problem.