Is it routine these days to "index-link" spousal maintenance/periodical payments- ie for the amount to rise in line with cost of living/RPI etc.
Why do some people get index-linking but others don't.
On what grounds would a request to index-link periodical payments be turned down by a judge/refused by the other side?
I don't know the full answer to your question but I would argue that index linking of SM can only apply if it is reasonable to assume that the financial source from which it stems is likely to rise in line with the index (of which the RPI is one. There are many indices so chose carefully).
If one has a fixed income then it would be unreasonable for it to be index linked.
As the party receiving SM works themselves into the labour market it could be argued that the SM should be lessened in real terms and so by fixing the amount, especially if it is for 5 years or so, reflects that gradual diminishing of need.
Where SM is payable for the duration of the life of a party the question of index linking is more pertinent and some provision for inflation likely.