DataStores and MemoryStore will stay separate data spaces. OrderedDataStores will not become obsolete as a result of MemoryStore but it may help you create periodic leaderboards more easily. Some experiences may choose to base their leaderboards in MemoryStoreService alone which is fine!
Just do be mindful of the per-experience quota and that you leave enough room for other use cases of MemoryStore if you base your leaderboards in MemoryStoreService. The quotas are very generous but if you abuse them too badly you might find yourself running out.
Always remember to remove data you no longer need, very important point to using this service.