Of course! And it can get even worse with a proprietary system, if the company goes bankrupt, then there's no system or code left at all... or they may open so

Of course! And it can get even worse with a proprietary system, if the company goes bankrupt, then there's no system or code left at all... or they may open source it. Although, the companies that open source their code after going bankrupt are extremely rare. But then there's the question, what will the clients do with that code? If it is a large system, it would take months to continue development on it - and who would pay for it? The most likely outcome is that the clients will go to the nearest competitor.