Magento update their version rapidly fast. we are using magento 220.127.116.11, we are moving our entire version from 18.104.22.168 to 2.1.7, after some months magento closed 2.1 series docs. Magento announce EoQ & EoS for 2.3 series. They update their versions quickly and without giving enough time to develop/upgrade own extensions. In our case we are using more than 15 custom modules, 25K products, 300 Categories and 50K customers. In future how magento stable their version?
Magento is rapidly upgrading it's versions to make sure they are modular and secure as per the Ecommerce standards and code standards. They have implemented Security Patches, PCI compliances, improved website browsing speed. It is adviceable to upgrade your extensions, theme to magento latest version, which would be good to secure your business. It is also recommended to keep upgrading your magento to make sure all the security patches are implemented to safe your website from hackers and malicious scripts. Once you are done with your Magento 1.9 upgradation to magento 2.3.x, implementing latest upcoming versions would not be that hard. just need to upgrade using command.
Magento is providing a tool to migrate your products, sales and customers from 1.9.x to 2.3.x. All your data can be migrated using this tool. https://devdocs.magento.com/guides/v2.3/migration/bk-migration-guide.html
but extensions and theme need to migrate. Even though there are tools which can migrate your extensions almost 70-80% but some manual intervention would still be needed. https://github.com/magento/code-migration
Magento has also launched Magento2 Commerce Cloud to give you a seamless deployment process, where you can focus on your business only and magento will take care of your Business Infra to give you scalability, 99% up time all the time.
To put it in a nutshell, It is highly recommended always to keep upgrading your magento2 version to the latest one to make sure your business is secure.