Thank you for this good question!
While the modern lightweight and mobile renderings become more and more popular, there is a good number of old projects that still use the classic rendering. We follow closely the trends and once the lightweight rendering usage exceeds 70-80% we will switch the default value of RenderMode to Auto.
Since this is a public forum thread, I strongly encourage everyone to share his/her opinion about changing the default value of this property.
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and share your thoughts.