Technically they can use any software to do the CMP-MPC
it's technically possible to do the process to access the members key
Emphasis mine, and that's my concern. It's
technically possible to set up this inheritance and recovery type of multi-sig arrangement yourself, but as you correctly point out, it is beyond the skill set of the vast majority of average users. It will also be beyond their skill set to recover their coins if your service disappears, and that's a very dangerous situation to be in. And you are not incentivized to release a tool to allow them to do so, since then they can easily bypass your pricing model.
the plan is not to let the users develop software, it's to provide another software to help them. A decentralized system allows a new software to do the job, unlike a centralized system rug.
You are essentially hoping that some unknown developer will be kind enough to develop a tool to allow users to recover their coins, for free, in their own time. That's a big assumption.
At first, Mixin Safe makes bitcoin multisig+timelock conveniently usable for average users, otherwise they have no other choices.
Second, we have been running for 6 years, that's long enough, we have no incentive to go offline.
Finally, even if we are offline, many developers or companies should be incentive enough to develop new tools. Imagine Mixin Safe can attract customers to pay $1000 per year, why no other companies want to do this? If we have 1000 customers, a new company can easily have 1000 customers to use their service.