The claim that Shopify cannot handle large catalogs is outdated. Stores with 10K, 50K and 100K SKUs run in production on Shopify today, and they run fast.
What actually changes at scale is not the platform but the migration. Variants, metafields and category structure need deliberate mapping. Done badly you end up with thousands of duplicate listings and a search index full of noise.
Magento gives you more raw flexibility. Shopify gives you a stable runway and a lower total cost of ownership. Above 10K SKUs the trade is usually worth it, provided the migration is engineered instead of improvised.