JL is struggling because it cannot sustain Waitrose losses.
Senior management incompetence only compounds the problem.
................and because the John Lewis departmental store business model is unsustainable long term.
In retail "losses" can occur for a number of reasons, but a key one is the cost of operating the store. In the case of departmental stores they are very large, multi-floor units with massive rents. They are also very expensive to staff, requiring a level of employees per floor to provide adequate customer service, meet H&S requirements, let alone good security - most important if a retail company want to limit "wastage" . To cover those costs you MUST generate profits ; if you fail to do that you are running a non-viable business, as Debenhams and House Of Fraser have cruelly found out.
No matter what "management" do, the market is dictating what happens finally to the retail business. No profit = no business. Hence the failures of Woolworth's, BHS, HMV, Maplin, ToysRus, etc, etc. The market rules, with the customer driving it with what they want in terms of price and physical shops, or simply an online site with a massive range all delivered to your door. This is the 21st Century, and the retailing I and my parents once knew, is going fast.
You can be the best manager in the world but you will not buck this trend. It is unstoppable and the only thing the owners / shareholders of these affected businesses can do is to pull out, take the smallest loss possible, and then invest elsewhere.