The effects of the mass layoffs and semi-forced resignations might not show up immediately. But they're running in just keep the lights on mode for sure. When it will really show up is when they now have a major problem (being hacked, major infrastructure going down, etc.) and it takes them a lot longer than it should to recover. That might be next week or in 6 months, but there's a good chance it will happen. Also important to note that Twitter Engineers that have left have described it as a "house of cards". All it will take is them falling behind on security updates being short-staffed now and I wouldn't be surprised to see them get hit with a major intrusion.
But even before that advertisers aren't going to want to go near Twitter for quite a while. Brand risk is more important than anything to most big companies and there are plenty of safer places for that right now. And when the inevitable long outage now comes that might be the nail in the coffin.
Updating also because it's being reported that 75% of the *remaining* employees opted to take severance after yesterday's ultimatum. So a company with 7,500 employees 2 weeks ago is now 900. GL with that. I mean they can send everyone home and the site will still run...just not for very long.