These sorts of issues plague a lot of bike designs where they try to do something radically different. The reason why the derailleur and chains are used so often is because they are relatively cheap, work well and require maybe a few cheap tools to remove/refit parts (I have done this myself many times).
Also as for the modular design. Why would I care about a modular design if I wanted something like this. You aren't going to be a courier one day and then suddenly running Gelato stand the next.