50 years would eliminate 99% of slave contracts in which slaves would benefit given the life expectancy - kind of how social security was praised as being good for the economy (since 90% of people did not live to be hexagenerians during that time, and therefore very few had to be paid what they put into the system.) Of course, ignoring culture this translates into English as "forever." But, the perpetuity lasted a lifetime (in context,) which is why foreigners were also inheritances to the children of Hebrews. If a Hebrew father bought slaves, he may die before 50 years - in which case a foreign slave becomes the inheritance of the children.
Slave contacts weren't for a 50 year duration, rather Jubilee was at 50 year intervals. Given that fact a significantly higher proportion of slaves would live to experience a jubilee year.
And, for some reason people are forgetting to take in context that Hebrews were not going around buying/stealing foreign slaves all of the time, or even some of the time. A slave/servant with an Hebrew master had to volunteer, or be a spoil. The Hebrews were not reavers; they were not so-called conquering imperialists invading nations on a whim (they would if you eat their children, or rape their family members continually.) So, the amount of foreign slaves Hebrews had from spoil was not very large. In fact, it was the family of those foreign slaves that constituted the "pool" of slaves for Hebrews. And, remember: when God gave the order to go to war, it usually meant they killed everyone. They weren't in the business of keeping the same entities with whom they warred - for good reason.
The fact that these laws are given at Sinai, before the conquest of Canaan, we could indeed view the Israelites as conquering imperialists. There was plenty of spoils to come in the near future.
That is not even incorporating the fact that it is still being ignored that "slavery," as it were, was an economy that is tantamount to work. (That particular bit is most hard for people to swallow because it means they are technically slaves themselves - and that they are calling kettles black by admonishing the same system of economy from antiquity they may extol in modernity.)
Why then does the scripture make a distinction between a hired worker and a slave? Clearly not everyone who labors is considered a slave.
Leviticus 25:39-40 If one of your countrymen becomes poor among you and sells himself to you, do not make him work as a slave. He is to be treated as a hired worker
What then, you may ask, is the distinction between a hired worked and a slave? The same chapter tells us
Leviticus 25:44-45 Your male and female slaves are to come from the nations around you; from them you may buy slaves. You may also buy some of the temporary residents living among you and members of their clans born in your country, and they will become your property.
The distinction back then is the same as it is now. Slaves are property, hired workers are not.
Let's look at Leviticus 25 in some more context
1 And the Lord spake unto Moses in mount Sinai, saying,
2 Speak unto the children of Israel,
Firstly, let's acknowledge that the chapter is addressed to the Israelites.
6 And the sabbath of the land shall be meat for you; for thee, and for thy servant, and for thy maid, and for thy hired servant, and for thy stranger that sojourneth with thee.
Foreigners were considered strangers and sojourners in the land. They couldn't own property permanently, they could only lease with the price being determined by the number of years until jubilee. This ensured that the land stayed under the traditional ownership of the 12 tribes.
10 And ye shall hallow the fiftieth year, and proclaim liberty throughout all the land unto all the inhabitants thereof: it shall be a jubile unto you; and ye shall return every man unto his possession, and ye shall return every man unto his family.
Here the word inhabitants is referring to the 12 tribes to whom God is giving the promised land to inhabit, not strangers or sojourners among them who weren't considered inhabitants. The very next sentence states "it shall be a jubilee into you". 'You' in this case is referring to the children of Israel being addressed in this chapter as stated in verse 2.
Every man returning to his possession and family again is referring to the Israelites, as foreigners didn't have possession in the land, and slaves didn't have family to return to.
Leviticus 25:39-55 then gets into the specifics. It gives three classes of slaves/servants and specific rules for each class. Hebrews who have sold themselves to Hebrews go free, and interestingly some believe this includes those who have voluntarily become slaves for life, the ones that have had their ear pierced. Hebrews who have sold themselves to foreigners go free if their relatives have failed to redeem them. Foreigners owned as slaves by Hebrews do not go free.
Furthermore foreigners debts were also not cancelled, Deuteronomy 15:1-3, so why would they set foreign slaves free?