Workarounds are always temporary. Over time, however, some may become a little more permanent than others.
Why build shared billing and account management service at all? There're the likes of Stripe and PayPal, BTC payment server, Electrum wallet and the standard desktop Monero-wallet after all.
Yes, my main man. However, over time multiple independent products will often end up needing the same: subscriptions, invoices, payment tracking, access management based on user's current plan, and quotas of a plan, tracking of these, account upgrades, refunds, and all the other pieces that quietly accumulate around billing. Rebuilding and maintaining the same functionality for every project will waste one's time, won't it?
One may build the required functionality for 1 or 2 his SaaS projects relatively easily. What if, however, there gets built a 3rd and 4th SaaS projects? A 5th and 6th ones? And several direct clients as well which would need to get billed as well be it reccurent or one-time payments? Repetition of functionanlity, which is custom one its own, then becomes unavoidable. And over time bugs always get discovered, functionality always needs some tweaks...
No software is ever finished.
The objective is straightforward: a dependable billing, account and access management service under one umbrella.