Is this a problem? Personally, i think so - for capping to replace
pre-buying of period tickets, it has to guarantee to never be more
expensive, otherwise people won't trust it. Some may feel that as long as
the system is transparent and predictable, it doesn't matter if it's not
always perfectly cost-efficient - if it's going to shaft you, you'll know,
and you can buy a ticket upfront to defeat it.

I guess the key point is whether a capping or other usage based discounting
system would run alongside period tickets or if it would eventually replace
some or all of them. In the longer term would period tickets even be
necessary? It seems to me a complete change to the fares model can be
achieved once paper tickets are eliminated.