  To correctly model the abundances of interstellar complex organic molecules
(iCOMS) in different environments, both formation and destruction routes should
be appropriately accounted for. While several scenarios have been explored for
the formation of iCOMs via grain and gas-phase processes, much less work has
been devoted to understanding the relevant destruction pathways, with special
reference to (dissociative) charge exchange or proton transfer reactions with
abundant atomic and molecular ions such as He$^+$, H$_3^+$ and HCO$^+$. By
using a combined experimental and theoretical methodology we provide new values
for the rate coefficients and branching ratios (BRs) of the reactions of He$^+$
ions with two important iCOMs, namely dimethyl ether (DME) and methyl formate
(MF). We also review the destruction routes of DME and MF by other two abundant
ions, namely H$_3^+$ and HCO$^+$. Based on our recent laboratory measurements
of cross sections and BRs for the DME/MF + He$^+$ reactions over a wide
collision energy range, we extend our theoretical insights on the selectivity
of the microscopic dynamics to calculate the rate coefficients $k(T)$ in the
temperature range from 10 to 298 K. We implement these new and revised kinetic
data in a general model of cold and warm gas, simulating environments where DME
and MF have been detected. Due to stereodynamical effects present at low
collision energies, the rate coefficients, BRs and temperature dependences here
proposed differ substantially from those reported in KIDA and UDfA, two of the
most widely used astrochemical databases. These revised rates impact the
predicted abundances of DME and MF, with variations up to 40% in cold gases and
physical conditions similar to those present in prestellar cores
