Who know what might happen in the future?
4 suppliers have gone under this month (first PfP Energy and MoneyPlus Energy, then UtilityPoint and People’s Energy). OFGEM moved customers of the first two to British Gas, and EDF for the third - an announcement for the last will probably appear in the next day or two (People’s Energy were larger than the other three).
The key point to note is that any credit balances those customers had with their defunct supplier gets transferred across to the new OFGEM appointed replacement supplier, and they’ll be set up with new accounts and tariffs.
Once the process is complete, customers are free to switch if they’re not satisfied with OFGEM’s choice.
As for Bulb, although I believe you’re right that they don’t generate their own power, they’re probably less at risk than some similar companies as they [Bulb] don’t offer fixed rate tariffs, so if wholesale prices continue to rise, they can pass those rises on when necessary (as long as they’re within OFGEM’s price cap).
EDIT: BTW, I was with PfP, and am being moved to British Gas. Last year, I had just moved to Yorkshire Energy, and they collapsed, and I got moved to Scottish Power. It’s not actually long since I moved off SP to PfP earlier this year! I’ve not lost any money as a consequence, but do find it rather a faff having to keep reviewing my energy supply arrangements!