For most applications, garbage collection.
For a while I was thinking of the question of "Why didn't production rules (a.k.a. expert systems, business rules engines) catch on as a general approach to programming?" Systems like that can do some remarkable things, similar to Excel and logic programming in that you can write programs that aren't concerned with the exact sequence of events that things have to be done in. This is not only convenient for the non-professional programmer, it makes some "chicken-and-egg" problems in conventional programming languages trivial, back in the 1980s people would have seen this to answer to parallel programming, today we might see it as an answer to the async choreography hell of front end programming.
The thing is that sometimes the order of events matters and when you look at rules engines they never settled on a standardized way to control execution. There are numerous things that work, an advanced rules engine implements several, but none of them are 100% satisfactory for everyone.
One answer to these problems I see is "rules and schemes" where there are two programs that exist side-by-side, one that defines what is to be done and the other that fills in the details of how exactly it is done.
I can see something like that for the general programming language case. (Write a "style sheet" that explains how memory management is done) Also sometimes I fantasize that the way to achieve what Rust is trying to achieve is some combination of
(1) A really good macro assembler
(2) A theorem prover
(3) An ergonomic parser generator that makes writing rich DSLs easy and completely mainstream.
(1) is a good project (e.g.
gas is by and for people who hate assembly language), (3) is a good project (I can rant about how bad parser generators hold developers back), the (1)-(2) punch is appealing with the the caveat that portability and optimization are problems in that case, but those could be addressed by developing architecture-specialized macro packs and
(4) A general-purpose superoptimizer
which hooks into (2) which is itself a project the world needs.