I've been thinking about this.
One solution is to change SC to two concurrent programmes.
First being lifetime membership (perhaps LT silver for 200 000 SCs and LT gold for 500 000 SCs) for accumulative credit totals.
Second being a change to SCs to similar basis as FF points. Have them non-expiring but deductible for the status tier achieved each year.
So for me, where I might earn 25k SCs per year, 20k surrendered per annum for silver and the balance being "banked", so each 6 years or so, I get gold for a year as a "thankyou" benefit. It'd also mean those gold tier members could retire from their working career with perhaps a bank of maybe 300 000 SCs to allow them to retain gold for at least another 6 years of their less hectic "retired" life (and possibly more depending on retired life flying committment), also as a thankyou benefit.
It would have a benefit to DJ of attracting more brand loyalty from those red tier members who may only earn say 5k SCs a year, but after 4 years they'd be rewarded for their loyalty by gaining silver for a year. Young flyers could also start to build their bank of SCs and hence bring them into the DJ "family" from a very early age, promoting lifetime loyalty rather then "best on day" shopping.
.....brand loyalty achieved, new flyer recruitment achieved and member rewards achieved no matter the tier. I can't see this "cheapening" the product at all because the upgrades will be more akin to "taster" programmes and the flyer that only earns 5k SCs a year won't ever achieve a LT gold for example....that will still be reserved only for the big ticket flyers.