It is very complicated to write your own correlation rules without the help of Splunk support. What Splunk could do better is to create an API to the standard SIEM tools, such as Microsoft Sentinel. The idea would be to make it less painful. In ELK Stack, Kibana is the query language with which you can search log files. I believe Splunk has also a query language in which they search their log files, but once you have identified the log file that you want to use for further security correlation, you want to very quickly transport that into your SIEM tool, such as Microsoft Sentinel. That is something that Splunk could make a little bit less painful because it is a lot of effort to find that log file and forward it. An API with Microsoft Sentinel or a similar SIEM tool would be a good idea.
