Well if you hate Siri so much there’s little incentive to stay within Apple environment. Smartphone is only little part of digital ecosystem nowadays. By “locking in” all your products you get the most of your purchase.
On the other hand, mix and matching everything will end you up with almost nothing. Spotify works relatively well with Google Assistant and their smart speakers, but iPhone might not work too well with those. I mean can you imagine someone using iPhone and Google Home Max? Sure it works if you download another google app. But you're just hindering the potential for both devices.
To say that if you don’t use Siri there is little incentive to stay within the Apple eco-system is ridiculous. I’ve tried Siri several times and given up on it because it’s so bad. But it’s just one app. I use my iPhone, iPad and Macs every day very happily without Siri. Locking in all your Apple products just leaves you with a poor user experience. I buy eBooks from Amazon, I watch video using Amazon Prime, I use Deezer because you can stream Hi-Resolution music unlike Apple Music. They all work absolutely fine on my Apple products.
If you stay locked in to Apple products you’re sacrificing quality for convenience. I accept that a lot of people will be happy with that but a lot of us are also happy to go for a mixed environment.