Some things to consider:
You wouldn't add a steel panel to an aluminum wall.
Aluminum is more expensive than steel.
A steel pool with a stainless steel or aluminum panel should outlast a regular steel pool. However, only 4 ft. of your wall is being protected by the panel while the rest is still steel and can easily rust if a pinhole leak develops. The problem is, the leak will be between the wall and the liner and you'll probably never notice it until the problem becomes too severe.

Just wanted to let you know my thoughts on walls. I'm in the pool industry and have seen an uncountable number of steel pools fail, many with stainless steel panels installed.

Josh