Will future apps be built with no database but only Redis (or other key-value stores) as a persistence layer?

SQL and NoSQL make the database spectrum. It's either SQL or not, i.e. NoSQL, so if the apps will be built without either, that means they will not have any persistance Redis is a DB, it's a key-value store. It also handles lists, sets, hashes so it's quite versatile. Redis' strength and weakness is the fact it's in memory, so it requires a different approach than "let's shove everything in here and worry about it later". There is absolutely no reason Redis can't be used as a sole data store as long as it's managed correctly. In a lot of cases it solves problems, in some cases causes them, but that's no different than any other system. One main point to realize is that whether it is SQL or NoSQL or SomeOtherSQLish thing. Use what's right for you and what fits your data. The reason K/V, Object, Document, etc stores exist is because a monopolistic SQL solution wasn't cutting it. Not that there was anything inherently wrong with SQL, not because someone someone wanted to make Frankenstein, it's because they just thought their data better fit into a non-relational structure.

In general, no, simplistic data stores like Redis will not replace databases (of any kind).  Every non-trivial application will probably need to deal with persistent data using a variety of data/operation and consistency models - from eventually consistent get/put on whole objects to strongly consistent transactions using a single field in millions of rows/objects whatever. Key/value stores will have their place.  So will document stores, "wide column" stores, object stores, even block stores and file systems.  A single application is likely to use many of these, either deployed solely for its own benefit or consumed as multi-tenant services.  It's called "polyglot persistence" and that's what will replace the "single source of truth" model for data storage in older applications.

Redis, MongoDB and other NO SQL DBs do not strictly follow ACID properties of the Database. We basically use Redis , MongoDB in cases where we just need to store Information as key-value pair.  Some of the example could be apps which just store information on 1 time basic and overwrite it when the new value come for the same key. I personally use Redis in one of my projects where i store RTT [ Round Trip Time ] between 2 routers per flow. Here the flow-id is the key. Now coming to RDBMS , we cannot completely remove them for persistence layer. SQL Server , Oracle DB etc are the heart of any Web Application where all the data resides. So any app , which is a wrapper over a big software will also use this DB to pull data. So yes we are seeing the increase use of "No SQL DB" , but RDBMS will not be completely wiped out.