Modern random access protocols are emerging as an efficient yet simple solution for arising internet of things (IoT) applications in upcoming beyond-5G systems. In this context, both terrestrial and non-terrestrial scenarios can benefit from the presence of multiple low-complexity receivers that act as relays, collecting packets from users and forwarding them towards a central unit. To gain insights on the potential of these setups, we investigate a two-tier slotted ALOHA (SA) multiple-relay system under an erasure channel model. We derive exact end-to-end throughput expressions for an arbitrary number of receivers, and complement our study by discussing the impact of channel impairments. The non-trivial outcome that adding relays is not always beneficial is highlighted and thoughtfully discussed.