The universe is flat to better than half a percent, and in ΛCDM that flatness is the residue of the most extreme fine-tuning in physics. Spatial curvature is gravitationally unstable under standard expansion: any departure from critical density grows relentlessly, so the measured |Omega_k| < 0.005 today (Planck 2018 with BAO) requires the energy density at the Planck epoch to have matched critical density to one part in 10^60. Run the film backward and the initial conditions must be balanced on a knife edge sixty decimal places deep; nothing in the standard framework supplies the balancing, the hot big bang simply must have begun that way.
Inflation was invented largely for this problem, and its solution is real but rented: a phase of accelerated expansion drives curvature toward zero exponentially, converting the fine-tuning into a prediction, provided the inflaton field exists, its potential has the required flatness, inflation lasts the required sixty e-folds, and it begins from a patch already smooth enough to inflate. Each requirement is itself unexplained: the field has never been identified, the potential is tuned by hand, the initial smooth patch re-imports a version of the problem inflation was hired to solve (the Penrose initial-conditions objection), and the trans-Planckian and measure problems attach to the mechanism structurally. The flatness problem is not so much solved as moved into the inflaton sector and renamed.
The standing is foundational: flatness is among the most precisely confirmed facts in cosmology, the only standard explanation depends on an undiscovered field with tuned properties, and any framework deriving near-flatness from ordinary dynamics, without a designed inflationary epoch, removes one of inflation's two remaining jobs.