> interference is considered as noise, which means that SNR simply drops
You're arguing that PHY data rates are low because SNR is low, and SNR is low because interference is high. In my experience, this is not the case. PHY rates can be quite high, but the overall throughput is low due to inefficient channel access at the MAC layer.
Wi-Fi doesn't consider interference as noise. Since it uses CSMA/CA to manage channel access, a device can only transmit if no other interfering device is transmitting, or if background interference is very low. This is why your device can be operating at a high MCS, but actual throughput will be so much lower. If your devices are using a low MCS, it's more likely that they're getting a weak signal from the access point.
In my experience WiFi clients at 2.4GHz never end up using the high MCS indices whenever they transmit (whenever the channel is clear). This has something to do with the fact that the CCA thresholds are fairly high, and that high SNR (30-50dB) is required for activating the high MCS indices[1]. I don't think these SNRs are achievable in a typical setting.
It is true that MAC backoffs are also a contributing factor to the throughput being low. The CCA assessment procedure detects both wifi preambles and non-WiFi interference (pure energy detection), which is why I say interference is modeled as noise. CCA does not for example, have some intelligent coexistence algorithm for dealing with zigbee or LTE-U or other ISM traffic.
You're arguing that PHY data rates are low because SNR is low, and SNR is low because interference is high. In my experience, this is not the case. PHY rates can be quite high, but the overall throughput is low due to inefficient channel access at the MAC layer.
Wi-Fi doesn't consider interference as noise. Since it uses CSMA/CA to manage channel access, a device can only transmit if no other interfering device is transmitting, or if background interference is very low. This is why your device can be operating at a high MCS, but actual throughput will be so much lower. If your devices are using a low MCS, it's more likely that they're getting a weak signal from the access point.