"Importantly it is a singleton. This implies that we will only be able to run one instance of it to ensure that all players are seeing the most up-to-date and correct games every time," the devs wrote. "We have optimized this service in D2R Ladder Items several ways in order to better match the current technology, but as have previously stated, many of the issues we have come from game development."
In the meantime, there's a selection of temporary solutions and roadmaps to rewrite Diablo 2's model so that it can better scale for modern demand. The service that just provides players with a list of games players, for instance is now being restructured into a separate service that is its own.
The developers are also planning to introduce an account queue, similar to World of Warcraft, to avoid situations in which servers become overwhelmed when hundreds of thousands of game instances are launched all simultaneously:
To help with this our staff are working in a login queue much like you may have encountered during World of Warcraft. This will ensure that the player count stays at a safe level, which is what we are currently at and allow us to monitor what the system is doing and take action before it brings the game to a complete stop.
Each time we solve a strain, we'll be capable of increasing the number of limits. This login queue has already been implemented in part in the backend (right moment Buy Diablo 2 Resurrected Items, it appears like an authentication error in your client) and should be fully implemented over the next few days for PCs, with consoles to follow after.