❶ - Just to be clear and in regards to what Henna said and to refute her statement, the same word for eternity was used in the following ayah (nothing to do with her huqb analysis of a random unrelated ayah and unrelated word as per this discussion):
وَمَا جَعَلْنَا لِبَشَرٍ مِّن قَبْلِكَ الْخُلْدَ ۖ أَفَإِن مِّتَّ
فَهُمُ الْخَالِدُونَ - "And We granted not to any human being
immortality before you (O Muhammad), then if you die, would they live
This is a significant ayah to understand this whole issue. Briefly, as a context, in this ayah 'Allah is consoling the Prophet that if he were to die, the disbelievers also would one day die, and be brought to account. They would not live forever to continue their disbelief without facing justice and having to answer for the truth they rejected which was brought by the Prophet. So even if you were to die, it does not mean they have been victorious. Indeed, they will also die one day, sooner or later, and be questioned for their rejection of truth. We should also remember many of the disbelievers were waiting for difficulties to come to the Prophet, and were also trying to kill him, thinking that will end the message and truth he brought, but Allah informs them and us through this ayah that the death of the Prophet will not mean the disbelievers have escaped the punishment that is to come'. It is important to understand this context as it relates to the issue on hand: eternity.
Highlighted in bold text indicates the translations used. But regardless of the translations, the context of the ayah indicates eternal life, as Allah is contrasting the death of the Prophet and saying "will the disbelievers not die? Will your death save them...." etc. Now looking at the verse on riba:
وَمَنْ عَادَ فَأُولَٰئِكَ أَصْحَابُ النَّارِ ۖ هُمْ فِيهَا
خَالِدُونَ... - "...But whoever returns to [dealing in interest or usury] - those are the companions of the Fire; they will abide
eternally therein." (2:275)
Taking both these verses indicates eternity, similar to the verses on disbelievers being eternally in Hell-fire. And likewise are others mentioned similarly as being in Hell-fire to abide therein. One example just so we are all up to speed and clear:
إِنَّ الْمُجْرِمِينَ فِي عَذَابِ جَهَنَّمَ خَالِدُونَ - "Verily, the
Mujrimûn (criminals, sinners, disbelievers, etc.) will be in the
torment of Hell to abide therein forever." (43:74)
❷ - So, now we are rightly faced with a problem, which is the whole reason of your question. Riba is a sin which is not shirk and not kufr, and so can criminality be (not shirk or kufr, although it can also indicate the criminality of shirk and kufr). Yet, we have eternity in Hell being mentioned, so to clear this confusion, we go to the sources of tafseer to seek an explanation. The first and highest means of tafseer is to use the Qur'an to explain the Qur'an, and when we scan the whole of the Qur'an (and the Sunnah), we find one explicit and clear and repeatedly mentioned reason for the eternal punishment of Hell, and that is shirk. One example:
إِنَّ اللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَاءُ ۚ وَمَن يُشْرِكْ بِاللَّهِ فَقَدِ افْتَرَىٰ إِثْمًا عَظِيمًا - "Verily, Allâh forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allâh in worship, he has indeed invented a tremendous sin." (4:48)
❸ - Following on from the above, and this is why people confuse such issues because it is built on a structure which you have to understand before going to the next, we have a situation where the only way to interpret the ayat on eternal Hell is in the context of shirk, whether that shirk is mentioned explicitly or not. Eg, those criminals who will be eternally in Hell must be there because they committed shirk (NOT that all criminals are mushrik but those who are mushrik, will be in Hell eternally). Same for those in riba (if they have committed shirk while consuming riba, eg they have legislated in their minds that shirk is halal as opposed to accepting it as a sin, such will be eternally in Hell). All this is on account of the shirk they committed, not the sin of criminality or riba as such.
❹ - Now we come to the real question upon which all the above is based:
"If only Shirk is unforgivable, does that by default mean that any
non-Muslim who doesn't commit Shirk can be forgiven and enter Jannah
(Paradise)? Not all Kuffar (unbelievers) commit Shirk."
I will not discuss this due to the fitnah it can cause, and also because I am only a student of the Deen. However, what is to be investigated in this regard is what really is shirk, how it is caused, and further, the hidden causes of shirk and even minor shirk (which some scholars have stated will be treated as major shirk, on account of the Qur'an and Sunnah never indicating separate/different punishments for minor and major shirk, as per their understanding). Once we understand the real nature of shirk, we will be better able to see whether there are kuffar who might be outside of the scope of shirk or not, and in what levels. Perhaps due to the complexity of this issue and that there are derivations needed, and being a very sensitive issue with profound implications, the scholars have not made a blueprint on exact point by point calculations. Instead, many seem to have said kufr and shirk is interchangeable, and in many cases, it might be the case. As for specifics, it may be that we have not been given complete knowledge over it. I have my own views, which like I said I won't share here in the open due to the fitnah it may cause.