Answer to your question:
Q1: Benefits of changing the Old Publishing model to the new Micro service-based publishing model.
A1: SDL Tridion Sites 8.0+ (formally SDL Web) introduces a new publishing framework to replace the (now deprecated) existing publishing framework. The new publishing framework improves on the deprecated existing framework in a number of ways
The old publishing framework requires you to configure the server name, port number, and other machine-specific information in Content Manager Explorer, which then stores the data in the Content Manager database. By contrast, the new publishing framework associates your Content Manager only with logical (abstract) publishing destinations (Target Types) such as "Live" or "Staging" or "Legally Compliant Server Farm in Germany", which are themselves organized in one or more Business Process Types. Other publishing concepts such as Publication Targets and Protocol Schemas no longer exist in the new framework.
In the new framework, you map such logical items to physical machines (which may be geographically distributed) to a new, separate part of the product called the Topology Manager. The Topology Manager does have limited graphical user interface: system administrators manage the infrastructure using a set of self-documenting PowerShell scripts, which lend themselves to the automation of the management of the infrastructure.
This new setup makes it much easier to manage your delivery infrastructure. Also, because the Topology Manager has its own database, the migration of content can now easily be separated from the migration of infrastructure information.
Q2: Can I still use DWT on SDL site 9.1?
Q3: Can I use microservices to publish on the file system?
A3: Yes, It's possible in the deployer microservice to store the published items in file system.
Q4: Is it really worth it if we publish on the file system and use DWT and user Tridion SIte 9?
A4: I can't say it's worth it or not, It's up to your business decision.
Upgrade to sites 9.1:
- SDL Tridion Sites 8.1 Standard Support already dropped
- Sites 9.1 is an improved version of Sites 8.1, It has lot of new features
- Sites 9.1 EOL until Dec 2022.
- My recommendation for the use of Topology manager in the present upgrade exercise as a step in the direction of future upgrades. The topology manager creates an extra component that enables the decoupling of the content manager from the content delivery divisions of SDL Tridion Sites 9.1. The content manager in SDL Tridion Sites 9.1 is dependent on Topology Manager as a part of its functional prerequisites. This means if used or not Topology Manager will always be installed, including its own database.
- Also, use the CIL to your web app to talking to the broker database not recommended in-process API
After the upgrade worth thinks to move away from the File system and move to a database to get used to the GraphQL feature to get the content in a headless way for modern development.
SDL Tridion Sites 9.5 planned to release end of august, worth thinking to migrate to 9.5 instead of sites 9.1
I hope it helps.