The basic failure behind the level of C172 sales is the failure to produce a better product at a lower price, so that more end-users can afford the item.
I do not know of any manufacturers that wouldn't like to sell a lot more of their product. More of their product out in the field means more profits from both production, as well as spare parts sales.
I personally find it hard to believe that certification costs and liability insurance costs are totally restricting any increase in C172 sales.
The major restriction on the sales level of C172's is purely the high purchase cost. If Cessna were seriously interested in increasing the production levels of the C172, they would spend some money on finding ways to lower production costs and purchase cost.
I believe Cessna have no interest in producing more C172's, they are content to sell a minimum number of them at high prices to a range of well-heeled clients to whom cost is no object, and Cessna are happy to keep the status quo and all their current production facilities and methods in place, to ensure good levels of profits from facilities whose setup costs were amortised decades ago.
What is needed is a 21st century disruptor, an "upstart" light aircraft manufacturer with a revolutionary new approach, to shake Cessna to the core, and to get them outside their carefully nurtured comfort zone.